FAM. 14. _Aciculidae._--Ctenidium replaced by a pulmonary sac,

tentacles cylindrical, pointed at the end, eyes behind their base, foot

long and narrow; central tooth and lateral very similar, pinched in at

the sides, external marginal broad, edge finely pectinate; shell small,

acuminate, with a blunt spire, operculum corneous. Terrestrial only.

Tertiary----. Genus, _Acicula_ (= _Acme_).

FAM. 15. _Truncatellidae._--Ctenidium replaced by a pulmonary sac,

proboscis very long, eyes sessile, behind the base of the tentacles,

shell small, evenly cylindrical, apex truncated in the adult.

Eocene----. Genera: _Truncatella_ (subg., _Taheitia_, _Blanfordia_, and

_Tomichia_), _Geomelania_ (subg., _Chittya_ and _Blandiella_), _Cecina_

(?).

FAM. 16. _Rissoidae._--Eyes at the external base of the tentacles,

epipodium with filaments, operculigerous lobe with appendages;

central tooth pleated at the basal angles, lateral large, bluntly

multicuspid, marginals long, narrow, denticulate at the edge; shell

small, acuminate, often elaborately sculptured, mouth entire or

with a shallow canal, operculum corneous. Marine or brackish water.

Jurassic----. Principal genera: _Rissoa_ (subg., _Folinia_, _Onoba_,

_Alvania_, _Cingula_, _Nodulus_, _Anabathron_, _Fenella_, _Iravadia_,

and others), _Scaliola_ (shell agglutinating fragments of sand, etc.),

_Rissoina_ (lip thickened, operculum with an apophysis as in _Nerita_),

_Barleeia_, _Paryphostoma_ (Eocene).

FAM. 17. _Hydrobiidae._--Eyes at the outer base of the tentacles,

penis behind the right tentacle, prominent, operculigerous lobe

without filaments; radula rissoidan, central tooth often with basal

denticulations; shell more or less acuminate, small, aperture

entire, operculum corneous or calcareous. Brackish or fresh water.

Jurassic----. Principal genera: _Baicalia_, with its various sub-genera

(p. 290); _Pomatiopsis_, _Hydrobia_, _Bithynella_, _Micropyrgus_

(Tertiary), _Pyrgula_, _Emmericia_, _Benedictia_, _Lithoglyphus_,

_Tanganyicia_, _Limnotrochus_ (?), _Jullienia_, _Pachydrobia_,

_Potamopyrgus_, _Littorinida_, _Amnicola_, _Fluminicola_ (subg.,

_Gillia_, _Somatogyrus_), _Bithynia_, _Fossarulus_ (Tertiary),

_Stenothyra_.

FAM. 18. _Assimineidae._--Ctenidium replaced by a pulmonary sac,

no true tentacles, eye-peduncles long, retractile; radula that of

_Hydrobia_; shell small, conoidal, operculum corneous, nucleus

sub-lateral. Eocene----. Genera: _Assiminea_, _Acmella_.

FAM. 19. _Skeneidae._--Radula resembling that of _Hydrobia_; shell

very small, depressed, widely umbilicated, operculum corneous.

Pleistocene----. Single genus, _Skenea_.

FAM. 20. _Jeffreysiidae._--Mantle with two pointed ciliated appendages

in front, tentacles ciliated, eyes sessile, far behind the base of

the tentacles; marginal teeth sometimes absent; shell small, thin,

pellucid, whorls rather swollen, operculum with marginal nucleus,

divided by a rib on the inner face. Recent. Genera: _Jeffreysia_,

_Dardania_. Marine, living on algae.

FAM. 21. _Litiopidae._--Epipodium with cirrhi on each side,

operculigerous lobe with appendages; radula rissoidan; shell small,

conical, columella truncated, operculum corneous. Eocene----. Genera:

_Litiopa_, living on the Sargasso weed, suspended by a long filament;

_Alaba_, _Diala_.

FAM. 22. _Adeorbidae._--Radula essentially rissoidan; shell depressed,

circular or auriform, widely umbilicated, operculum corneous,

paucispiral, nucleus excentrical. Pliocene ----. Principal genera:

_Adeorbis_, _Stenotis_, _Megalomphalus_.

FAM. 23. _Viviparidae._--Snout blunt, tentacles long, right tentacle in

the male deformed, pierced with a hole corresponding to the aperture

of the penis, two cervical lobes, the right being siphonal, foot

with an anterior transverse groove; teeth broad, shallowly pectinate

at the ends; shell turbinate, whorls more or less rounded, aperture

continuous, operculum corneous, nucleus sub-lateral, with a false

sub-central nucleus on the external face. Animal ovoviviparous.

Fresh-water. Cretaceous ----. Genera: _Vivipara_ (= _Paludina_),

subg., _Cleopatra_, _Melantho_, _Tulotoma_; _Tylopoma_ (Tertiary), and

_Lioplax_.

FAM. 24. _Valvatidae._--Branchia exserted, bipectinate, carried on the

back of the neck, a filiform appendage (Fig. 66, p. 159) on the right

of the neck, penis under the right tentacle, prominent, eyes sessile,

behind the tentacles; radula like that of _Vivipara_; shell small,

turbinate or flattened, operculum corneous, nucleus central. Fresh

water. Jurassic ----. Single genus, _Valvata_.

FAM. 25. _Ampullariidae._--Snout with two tentacles, tentacles proper

very long, tapering, eyes prominently pedunculate, two cervical lobes,

the left siphonal, respiratory cavity divided by a partition, a large

branchia in the right chamber, the left functioning as a pulmonary

sac (Fig. 65, p. 158); radula large, central tooth multicuspid,

base broad, lateral and marginals falciform, simple or bicuspid;

shell large, turbinate or flattened, spire small, whorls rounded;

operculum generally corneous, nucleus sub-lateral, false nucleus as in

_Vivipara_. Fresh water. Cretaceous ----. Single genus, _Ampullaria_

(subg., _Ceratodes_, _Pachylabra_, _Asolene_, _Lanistes_, and

_Meladomus_).

FAM. 26. _Cerithiidae._--Branchial siphon present, short, eyes variable

in position; central tooth small, evenly cusped, lateral hollowed at

base, multicuspid, marginals narrow; shell long, turriculate, whorls

many, generally tuberculate, varicose or spiny, aperture sometimes

strongly channelled; operculum corneous, subcircular, nucleus

nearly central. Marine or brackish water. Trias ----. Principal

genera: _Triforis_, shell small, generally sinistral; _Fastigiella_,

_Cerithium_ (Fig. 12, p. 16), _Bittium_, _Potamides_ (subg.,

_Tympanotomus_, _Pyrazus_, _Pirenella_, _Telescopium_, _Cerithidea_,

_Lampania_, all brackish water), _Diastoma_ (Eocene), _Cerithiopsis_;

_Ceritella_ (Jurassic), _Brachytrema_ (Jurassic), and _Planaxis_

(subg., _Quoyia_ and _Holcostoma_).

FAM. 27. _Modulidae._--No siphon, radula of _Cerithium_; shell with

short spire, columella strongly toothed at the base, aperture nearly

circular. Recent. Single genus, _Modulus_.

FAM. 28. _Nerineidae_.--Shell solid, long, sub-cylindrical, aperture

channelled, columella and interior of whorls with continuous ridges,

extending up the spire. Genera: _Nerinea_ (Trias to Cretaceous),

_Aptyxiella_ (Jurassic).

FAM. 29. _Melaniidae._--Border of mantle festooned, foot broad, with

an anterior groove, penis present; radula closely resembling that of

_Cerithium_; shell long, spiral, with a thick periostracum, surface

with tubercles, ribs, or striae, suture shallow; operculum corneous,

paucispiral, nucleus excentrical. Animal ovoviviparous. Fresh water.

Cretaceous ----. Principal genera: _Melania_ (with many sections or

sub-genera), _Pachychilus_, _Claviger_ (= _Vibex_), _Hemisinus_,

_Pirena_, _Melanopsis_, _Tiphobia_, _Paludomus_ (subg., _Philopotamis_,

_Tanalia_, _Stomatodon_), _Hantkenia_ (Eocene), _Larina_ (?).

FAM. 30. _Pleuroceridae._--Mantle edge not festooned, no copulatory

organ, otherwise like Melaniidae; operculum with nucleus sub-marginal.

Animal oviparous. Fresh-water. Cretaceous ----. Genera: _Pleurocera_

(including _Io_, Fig. 12, p. 16, _Angitrema_, _Lithasia_,

_Strephobasis_), _Goniobasis_, _Anculotus_, _Gyrotoma_.

FAM. 31. _Pseudomelaniidae._--Shell resembling that of Melaniidae,

but marine. Genera: _Pseudomelania_, _Loxonema_, _Bourguetia_,

_Macrochilus_. Palaeozoic to Tertiary strata.

[Illustration: FIG. 275.--_Melania confusa_ Dohrn, Ceylon.]

FAM. 32. _Turritellidae._--Mantle with a siphonal fold on the right

side; radula variable (p. 224); shell long, whorls many, slowly

increasing in size, transversely ribbed or striated, aperture small;

operculum corneous, nucleus central. Jurassic ----. Principal genera:

_Turritella_, _Mesalia_, _Protoma_, _Mathilda_ (?).

FAM. 33. _Coecidae._--Tentacles long, eyes sessile at their base; shell

small, spiral in the young form, spire generally lost in the adult,

the shell becoming simply a straight or curved cylinder; operculum

corneous, multispiral. Eocene ----. Single genus, _Coecum_.

FAM. 34. _Vermetidae._--Visceral sac greatly produced, irregularly

spiral, no copulatory organs (radula, Fig. 126, p. 223), shell tubular,

irregularly coiled, last whorls often free, aperture circular;

operculum corneous, circular, nucleus central. Carboniferous ----.

Principal genera: _Vermetus_; _Siliquaria_ (Fig 153, p. 248), a long

fissure, or series of holes, runs along a considerable part of the

shell, operculum with outer face spiral, elevated.

FAM. 35. _Strombidae._--Foot narrow, arched, metapodium greatly

produced, snout long, eye peduncles long, thick, eyes elaborate, siphon

short, penis prominent, bifurcate; central tooth with strong median

cusp, marginals falciform, slender, edge more or less denticulate;

shell solid, spire conical, outer lip generally dilated into wings

or digitations, channelled before and behind, a labial sinus at the

base, distinct from the anterior canal; operculum small for the

aperture, corneous, claw-shaped, edge notched. Lias ----. Genera:

_Strombus_ (Fig. 99, p. 200); _Pereiraea_ (Miocene), _Pteroceras_

(Fig. 277; digitations of the outer lip very strong), _Rostellaria_

(spire produced, anterior canal very long), _Rimella_, _Pterodonta_,

_Terebellum_ (base of shell truncate, spire short).

[Illustration: FIG. 276.--Development of _Coecum_: =A=, showing

the gradual formation of septa; _a_, apex; _ap_, aperture; _ss_,

first septum; _s´s´_, second septum. (After de Folin.) =B=, adult

form of _C. eburneum_ Ad., Panama, x 10.]

FAM. 36. _Chenopodidae_ (= _Aporrhaidae_).--Foot flat; lateral and

marginal teeth not denticulate; shell resembling that of _Strombus_,

outer lip dilated, wing-like, no labial sinus. Jurassic ----. Genera:

_Chenopus_ (= _Aporrhais_, _Diastema_, _Malaptera_, _Harpagodes_,

_Alaria_) (last four from Secondary strata).

FAM. 37. _Struthiolariidae._--Radula allied to that of _Strombus_,

marginals occasionally multiplied; shell buccinoid, very solid, outer

lip thickened, canal short, operculum claw-shaped, notched, nucleus

terminal. Tertiary ----. Single genus, _Struthiolaria_ (subg.,

_Perissodonta_, marginal teeth multiplied).

FAM. 38. _Cypraeidae._--Mantle with two large lateral lobes reflected

and meeting over the shell, siphon small; central and lateral teeth

bluntly tricuspid or multicuspid, laterals fairly broad, edges cusped

or finely pectinate; shell polished, solid, spire generally concealed

in the adult or overlaid with enamel, aperture straight, narrow,

nearly as long as the shell, toothed at the sides, channelled at each

end, labium inflected; no operculum. Jurassic ----. Genera: _Ovula_

(including _Amphiperas_, _Transovula_, _Cyphoma_, _Radius_, _Simnia_),

_Pedicularia_, _Cypraea_ (with subg., _Cypraeovula_, _Cypraedia_, and

_Trivia_), and _Erato_.

FAM. 39. _Doliidae._--Foot expanded, wider and longer than the shell,

truncated and thickened in front, siphon very long and narrow; central

tooth with very strong median and small lateral and basal cusps,

lateral and marginals bluntly falciform; shell ventricose, without

varices, spire short, outer lip generally simple, anterior canal rather

wide, no operculum. Cretaceous ----. Genera: _Dolium_ (subg., _Malea_,

outer lip thickened, denticulate, reflected); _Pirula_, mantle with two

lateral lobes reflected over part of the shell, shell fig-shaped (Fig.

278).

[Illustration: FIG. 277.--Three stages in the growth of

_Pteroceras rugosum_ Sowb., E. Indies, showing the development of

the ‘fingers.’]

FAM. 40. _Cassididae._--Foot broad, siphon long (radula, Fig. 125, p.

223); shell ventricose, with varices, spire short, outer lip reflected

or thickened, anterior canal short, recurved narrow; operculum

semi-lunar, with ribs radiating from a marginal nucleus. Cretaceous

----. Genera: _Cassis_ (subg., _Semicassis_ and _Cypraecassis_),

_Morio_ (= _Cassidaria_), _Oniscia_.

FAM. 41. _Columbellinidae._--Shell solid, ribbed, usually cancellated,

with an oblique posterior canal, columella callous, more or less

reflected. Genera: _Columbellina_, _Columbellaria_, _Zittelia_,

_Petersia_, _Alariopsis_ (?). Secondary strata only.

FAM. 42. _Tritonidae_.--Foot short, narrow; siphon short, not

prominent; radula allied to that of Cassididae; shell thick, varicose;

outer lip inflected and thickened, canal long, periostracum often

thick and hairy, operculum corneous, nucleus terminal or sub-marginal.

Cretaceous ----. Genera: _Triton_ (Fig. 191, p. 275; subg.,

_Epidromus_, _Plesiotriton_, _Simpulum_, _Ranularia_, _Argobuccinum_);

_Persona_, aperture toothed, narrow; columella reflected upon the last

whorl; _Ranella_, shell dorso-ventrally compressed, generally with two

continuous lateral varices, posterior canal present.

The position of the following four families is doubtful:--

FAM. 43. _Oocorythidae._--Siphon short, foot broad, eyes absent, radula

taenioglossate; shell buccinoid or cassidiform, operculum corneous,

spiral. Recent. Single genus, _Oocorys_.

FAM. 44. _Subulitidae._--Shell elongate, fusiform, smooth; suture

shallow, base truncate or rounded, aperture channelled or notched.

Ordovician to Trias. Genera: _Subulites_, _Fusispira_, _Euchrysallis_.

[Illustration: FIG. 278.--_Pirula Dussumieri_ Val., Philippines.

× ½.]

FAM. 45. _Seguenziidae._--Radula taenioglossate, shell trochiform,

aperture channelled, columella twisted, operculum multispiral, nucleus

central. Pliocene ----. Single genus, _Seguenzia_.

FAM. 46. _Choristidae._--Anterior tentacles united by a frontal veil,

posterior simple; eyes absent, foot with tentaculae before and behind;

three central teeth, outer marginal with a basal plate; shell helicoid,

suture deep, peristome continuous, operculum corneous, paucispiral.

Pliocene ----. Single genus, _Choristes_.

SECTION II. HETEROPODA.--Foot fin-shaped, not flat.

The Heteropoda are free-swimming Mollusca, being, like the Pteropoda,

Gasteropoda modified to suit their pelagic environment. Their nervous

system is streptoneurous, and they are therefore probably derived from

the Prosobranchiata, but they are highly specialised forms. Pelseneer

considers them far more widely removed from the Streptoneura than the

Pteropoda are from the Euthyneura. They swim on the surface “upside

down,” _i.e._ with the ventral side uppermost.

The tissues and shell are transparent, permitting observation of the

internal organs. In the Pterotrachaeidae the foot takes the form

of a fan-shaped disc, usually furnished with a sucker. The body is

compressed at the posterior end, often with a ventral “fin.” In

_Atlanta_ the foot consists of three very distinct parts: a propodium,

a mesopodium, on which is a small sucker, and a metapodium, which

carries the operculum. The branchiae are carried on the visceral sac,

and are free in _Pterotrachaea_, slightly protected by the shell in

_Carinaria_, and entirely covered in _Atlanta_; absent altogether in

_Firoloida_.

The head carries two tentacles (except in _Pterotrachaea_),

with large, highly organised eyes on short lobes at their outer

base. The alimentary tract consists of a long protrusible proboscis,

with a taenioglossate radula (Fig. 132, p. 227), a long oesophagus,

and a slightly flexured intestine. In _Atlanta_ the visceral

sac is spiral and protected by a spiral planorbiform shell; in

_Carinaria_ the visceral sac is small, conical, protected by a

very thin capuliform shell. There is no shell in _Pterotrachaea_

or _Firoloida_.

The Heteropoda are dioecious. In the male there is a flagellum

behind the penis, which is near the middle of the right side.

_Pterotrachaea_ lays long chains of granular eggs, and has

been noticed to produce a metre’s length in a day. The eggs of

_Atlanta_ are isolated. The embryo has a deeply bilobed velum.

FAM. 1. _Pterotrachaeidae._--Body long, with a caudal “fin;” branchiae

dorsal, free or partly protected by a shell; foot consisting of a

muscular disc, with or without a sucker.

_Pterotrachaea_ proper has no mantle, shell, or tentacles. The

branchiae are disposed round the visceral sac, at the upper part of

which is the anus. In _Firoloida_ the body is abruptly truncated

behind, with a long filiform segmented caudal appendage; visceral sac

at the posterior end: fin-sucker present or absent in both male and

female. _Cardiapoda_ resembles _Carinaria_, but the visceral sac is

more posterior and is only slightly protected by a very small spiral

shell. _Carinaria_ (Fig. 279) has a rugose translucent skin, visceral

sac sub-median, apparently pedunculated, covered by a capuliform shell.

The larval shell, which persists in the adult, is helicoid.

FAM. 2. _Atlantidae._--Shell spiral, operculate, covering the animal.

Branchiae in a dorsal cavity of the mantle; foot trilobed, with a small

sucker on the mesopodium.

The shell of _Atlanta_ is discoidal and sharply keeled, while that of

_Oxygyrus_ is nautiloid, with the spire concealed, no keel, aperture

dilated.

(_c_) GYMNOGLOSSA.--Radula and jaws absent; proboscis prominent, sexes

probably separate, penis present. The section is probably artificial

and unnecessary, the families composing it being, in all probability,

Taenioglossa which have lost their radula in consequence of changed

conditions of life (pp. 79, 225).

[Illustration: FIG. 279.--_Carinaria mediterranea_ Lam., Naples:

_a_, anus; _br_, branchiae; _f_, foot; _i_, intestine; _m_,

mouth; _p_, penis; _s_, sucker; _sh_, shell; _t_, tentacles. × ½.]

FAM. 1. _Eulimidae._--Proboscis very long, retractile, mantle

forming a siphonal fold; shell small, long, subulate, polished;

suture shallow, aperture continuous, operculum present or absent.

Animal often parasitic, sucking the juices of its host by its long

proboscis. Trias----. Genera: _Eulima_ (subg., _Subularia_, _Arcuella_,

_Apicalia_, _Mucronalia_, _Stiliferina_, and others), _Stilifer_,

_Scalenostoma_, _Niso_, and _Hoplopteron_.

FAM. 2. _Pyramidellidae._--Tentacles auriform, proboscis as in

Eulimidae, a prominent _mentum_ or flap under the buccal orifice; shell

usually small, conical; suture shallow, apical whorls (the embryonic

shell) sinistral (p. 250), operculum corneous, paucispiral; nucleus

excentrical. Trias----. Genera: _Pyramidella_ (subg., _Syrnola_,

_Otopleura_, _Chrysallida_, _Mumiola_), _Odostomia_, _Eulimella_,

_Murchisoniella_, _Turbonilla_ (subg., _Dunkeria_ and _Cingulina_).

(_d_) RACHIGLOSSA (p. 220).--Proboscis long, retractile; siphon

distinct, radula without uncini, sometimes without laterals; teeth

strongly cusped; shell generally wholly external.

FAM. 1. _Muricidae._--Eyes sessile at the outer base of the tentacles,

penis large, behind the right tentacle, radula within the retractile

proboscis, central tooth (Fig. 119, p. 220) with at least three

strong cusps, laterals plain; shell solid, more or less tuberculate,

spiny and varicose, anterior canal varying from a mere notch to a

long channel. Cretaceous----. Principal genera: (i.) _Muricinae_,

nucleus of operculum sub-terminal; _Trophon_, _Typhis_, _Murex_ (with

many subdivisions), _Ocinebra_ (including _Cerastoma_, _Vitularia_,

and _Hadriania_), _Urosalpinx_, _Eupleura_, _Pseudomurea_. (ii.)

_Purpurinae_, nucleus of operculum lateral; _Rapana_ (including

_Latiaxis_), _Purpura_ (with subg., _Cuma_, _Iopas_, _Vexilla_, and

_Pinaxia_), _Monoceros_ (including _Chorus_), _Purpuroidea_ (Secondary

strata), _Pentadactylus_, _Sistrum_, _Concholepas_.

FAM. 2. _Coralliophilidae._--Animal living in Madrepores, resembling

_Purpura_, radula absent; shell variously shaped, often deformed

or tubular, operculum that of _Purpura_, if present. Miocene----.

Principal genera: _Rhizochilus_, _Coralliophila_, _Leptoconchus_,

_Magilus_ (Fig. 29, p. 75), _Rapa_.

FAM. 3. _Columbellidae._--(Radula, Fig. 123, p. 222.) Shell small,

solid, fusiform, aperture narrow, canal short, outer lip thickened.

Miocene----. Single genus, _Columbella_ (subg., _Nitidella_, _Anachis_,

_Meta_, _Strombina_, _Atilia_, _Conidea_, _Amphissa_, _Mitrella_, and

others).

FAM. 4. _Nassidae._--Foot long and broad, often with terminal

appendages; siphon long, eyes on outer base of tentacles, central

tooth of radula arched, multicuspid, lateral strongly bicuspid, with

small denticles between the cusps; shell rather small, buccinoid,

columella more or less callous, outer lip thickened, often toothed;

operculum corneous, edges often toothed. Miocene----. Principal genera:

_Nassa_ (with many sections), _Amycla_, _Desmoulea_, _Cyclonassa_,

_Canidia_ (subg., _Clea_ and _Nassodonta_), _Dorsanum_, _Bullia_ (=

_Buccinanops_, Fig. 62, p. 185), _Truncaria_.

FAM. 5. _Buccinidae._--Siphon rather long, eyes at outer base of

tentacles; central tooth of radula with 5 to 7 cusps, laterals

bicuspid or tricuspid (Fig. 118, p. 220); shell more or less fusiform,

thick, covered with a periostracum, canal of varying length, outer

lip simple or thickened; operculum corneous, nucleus variable in

position. Cretaceous----. Principal genera: Group i. _Chrysodomus_

(with sections _Neptunea_, _Volutopsis_, _Pyrolofusus_, _Jumala_),

subg., _Sipho_; _Siphonalia_ (subg., _Kelletia_). Group ii. _Liomesus_

(= _Buccinopsis_). Group iii. _Buccinum_ (Fig. 1 B, p. 6; subg.,

_Volutharpa_, _Neobuccinum_). Group iv. _Cominella_, _Tritonidea_,

_Pisania_, _Euthria_; _Anura_ (Miocene), _Genea_ (Pliocene), _Metula_,

_Engina_. Group v. _Phos_, _Hindsia_. Group vi. _Dipsaccus_ (=

_Eburna_), _Macron_. Group vii. _Pseudoliva_.

FAM. 6. _Turbinellidae._--Central tooth of radula tricuspid, median

cusp strong, lateral bicuspid, cusps unequal (Fig. 117, p. 220);

shell fusiform or pear-shaped, heavy, canal often long, operculum

corneous, claw-shaped, nucleus terminal. Miocene----. Principal

genera: _Turbinella_, _Cynodonta_, _Tudicla_ (subg., _Streptosiphon_);

_Piropsis_ (Cretaceous), _Perissolax_ (Cretaceous), _Strepsidura_

(Eocene, subg., _Whitneya_), _Melapium_, _Fulgur_ (= _Busycon_,

Fig. 150, p. 249, including _Sycotypus_), _Melongena_ (subg.,

_Pugilina_, _Myristica_); _Liostoma_ (Eocene), _Hemifusus_ (subg.,

_Megalatractus_), _Ptychatractus_, _Meyeria_.

[Illustration: FIG. 280.--_Turbinella pyrum_ Lam., Ceylon. × ⅔.]

FAM. 7. _Fasciolariidae._--Eyes at the outer base of the tentacles

(radula, Fig. 121, p. 221); shell fusiform, spire long, canal often

very long, columella often with a fold at the base; operculum corneous,

nucleus terminal. Cretaceous----. Principal genera: _Fusus_ (including

_Sinistralia_, _Aptyxis_, _Troschelia_), with subg., _Serrifusus_

(Cretaceous), _Clavella_ (subg. _Thersites_), _Fasciolaria_, _Latirus_

(subg. _Polygona_, _Peristernia_, _Leucozonia_, _Lagena_; _Mazzalina_

(Eocene), _Chascax_).

[Illustration: FIG. 281.--_Latirus_ (_Leucozonia_) _cingulatus_

Wood, Panama.]

FAM. 8. _Mitridae._--Siphon rather long, with anterior appendages,

eyes on the side of the tentacles, proboscis very long; radula

variable, laterals sometimes lost (Fig. 120, p. 221); shell fusiform,

solid, spire more or less pointed, columella with several prominent

folds, the posterior the largest, aperture rather narrow, no

operculum. Cretaceous----. Principal genera: _Mitra_ (with many

sections), subg., _Strigatella_, _Mitreola_, _Mutyca_, _Dibaphus_;

_Plochelaea_ (Tertiary), _Thala_; _Turricula_ (with several sections),

_Cylindromitra_, and _Imbricaria_.

FAM. 9. _Volutidae._--Foot broad in front, head laterally dilated

into lobes, on which are placed the sessile eyes; siphon prominent,

with appendages at the base (radula, Fig. 122, p. 221); shell thick,

often shining, fusiform, globular or cylindrical, columella projecting

anteriorly, with several folds, the anterior of which is the largest,

aperture notched, canal not produced, operculum generally absent.

Cretaceous----. Principal genera: _Cryptochorda_ (Eocene), _Zidona_,

_Provocator_, _Guivillea_, _Yetus_ (= _Cymbium_), _Voluta_ (with many

sections), _Volutolithes_ (chiefly Eocene), _Volutolyria_, _Lyria_,

_Enaeta_, _Volutomitra_.

FAM. 10. _Marginellidae._--Foot broad, siphon without appendages,

mantle largely reflected over the shell; radula without laterals,

central tooth comb-like, cusps rather blunt; shell oval or conoidal,

polished, aperture narrow, outer lip thickened, columella with many

folds; no operculum. Eocene----. Principal genera: _Marginella_, with

many sections and so-called sub-genera; _Persicula_, _Pachybathron_

(?), _Cystiscus_, _Microvoluta_.

[Illustration: FIG. 282.--_Voluta nivosa_ Lam., West Australia. ×

⅔.]

[Illustration: FIG. 283.--_Oliva porphyria_ Lam., Panama.]

FAM. 11. _Harpidae._--Foot large, with a transverse groove, separating

off a semi-lunar propodium; mantle partly reflected over the shell;

shell ventricose, polished; spire short, strongly longitudinally

ribbed, ribs prolonged over the suture, columella callous; no

operculum. Eocene----. Single genus, _Harpa_ (subg., _Silia_).

FAM. 12. _Olividae._--Propodium semi-lunar, with a longitudinal groove

above, mesopodium reflected laterally over the shell; central tooth of

radula tricuspid on a very broad base, lateral simple, hooked; shell

sub-cylindrical or fusiform, polished; aperture narrow, operculum

present or absent. Cretaceous----. Principal genera: _Oliva_ (Figs.

283, and 98, p. 199), _Olivancillaria_ (including _Lintricula_ and

_Agaronia_), _Olivella_, _Ancilla_ (subg., _Ancillina_).

(_e_) TOXOGLOSSA (p. 218).--Radula with normal formula 1·0·1, teeth

large; oesophagus with a large poison gland; animal carnivorous,

exclusively marine.

[Illustration: FIG. 284.--_Terebra subulata_ L., Ceylon.]

[Illustration: FIG. 285.--_Pleurotoma tigrina_ Lam., E. Indies.]

FAM. 1. _Terebridae._--Eyes at the end of the tentacles, shell

subulate, many whorled, operculum with terminal nucleus. Eocene----.

Single genus, _Terebra_, with several sections.

FAM. 2. _Conidae._--Eyes on outer side of tentacles, siphon prominent;

shell conical or fusiform, aperture narrow. Cretaceous----. Principal

genera: _Conus_, shell solid, spire short, aperture narrow, straight,

internal partitions partly absorbed; _Conorbis_, _Genotia_ (with

several sections, chiefly Tertiary), _Pusionella_, _Columbarium_,

_Clavatula_, _Surcula_, _Pleurotoma_; _Borsonia_ (Eocene), _Drillia_

(subg., _Spirotropis_), _Bela_, _Mangilia_ (including _Daphnella_,

_Clathurella_, and others), _Halia_.

FAM. 3. _Cancellariidae._--Proboscis short, usually no radula, shell

oval, columella strongly plicate; no operculum. Cretaceous----. Single

genus, _Cancellaria_ (subg., _Merica_, _Trigonostoma_, _Admete_).

CHAPTER XV

CLASS GASTEROPODA (_continued_): OPISTHOBRANCHIATA AND

PULMONATA

=Order III. Opisthobranchiata=

Visceral loop not twisted (except in _Actaeon_) in a figure of 8

(Euthyneurous type, p. 203), auricle usually behind the ventricle,

ctenidium often replaced by secondary branchiae, pallial cavity, if

existing, more or less open, shell present or absent, operculum absent

(except in _Actaeon_), animal hermaphrodite, with separate sexual

openings, marine only.--Carboniferous to present time.

The character of their nervous system decisively removes the

Opisthobranchiata from the Prosobranchiata, and approximates them to

the Pulmonata. _Actaeon_, however, which is streptoneurous, as well

as possessing an operculate shell with prominent spire, forms an

interesting link with the Prosobranchiata. At the opposite extreme

to _Actaeon_ stand forms like _Siphonaria_ and _Gadinia_, which are

probably close links with the Pulmonata (p. 19). The generative

system of the whole group, which is, as in the Basommatophora, of

the hermaphrodite type, without mutual fecundation, is another link

of connexion with the Pulmonata. The respiratory organs present the

most varied forms, sometimes consisting of one ctenidium (never two),

sometimes of secondary branchiae, variously placed, while sometimes no

special organ exists.

The prolongation of the foot into lateral epipodia or parapodia

(possibly to aid in swimming), and the effect of the epipodia upon the

shell, according as they involve it completely or partially, are among

the most instructive features of the Opisthobranchiata. If the epipodia

are developed on the anterior portion of the body, and do not become

reflected, they may, as in most Pteropoda Thecosomata, not directly

affect the shell. But when, as in the Tectibranchiata, the epipodia

are medio-lateral, and tend to envelope the shell, their effect may

be traced by a series of forms varying in proportion to the amount

of shell-surface covered by the epipodia. The two principal lines

along which modification takes place are the gradual reduction of the

spiral nature of the shell, and the gradual lessening of its solidity.

Both these changes are the direct result of the additional protection

afforded to the visceral mass by the reflected epipodia, which renders

the existence of a shell less and less necessary. A precisely similar

line of change is seen in the Pulmonata, culminating in forms like

_Arion_ (p. 174).

[Illustration: FIG. 286.--Illustrating the transition of form

in the shell of Tectibranchiata from the pointed spiral to the

almost flattened plate: =A=, _Actaeon_; =B=, _Aplustrum_; =C=,

_Cylichna_; =D=, _Atys_; =E=, _Philine_; =F=, _Dolabella_; =G=,

_Aplysia_; =H=, _Pleurobranchus_. (Not drawn to scale.)]

[Illustration: FIG. 287.--Illustrating the gradual covering of

the shell in the Tectibranchiata by the epipodia and mantle: =A=,

_Haminea_; =B=, _Scaphander_; =C=, _Aplustrum_; =D=, _Aplysia_;

=E=, _Philine_; _c.d_, cephalic disc; _ep_, _ep_, epipodia; _sh_,

shell. (Not drawn to scale.)]

The habits of life of the Opisthobranchiata are very varied. Some,

especially the heavier types, burrow in sand, and are then usually

furnished with a broad cephalic disc, as a digging apparatus; some

(certain _Bulla_) flit about in shallow pools on mud flats; others

(_Phyllirrhoe_ and the Pteropoda) swim freely in the open sea; others

(most Nudibranchiata) crawl slug-like on sea-weeds or corallines, and

in colour singularly harmonise with their environment (p. 71 f.);

others again (_Siphonaria_, _Gadinia_), stick limpet-like to rocks

between tide marks. As a rule, they occur only in clean salt water,

but _Embletonia_ has been found in the Victoria Docks at Rotherhithe,

as well as in parts of the Baltic, where the water has only 7 parts

of salt in 1000, while _Limapontia_ occurs in nearly fresh water at

Bornholm and Gothland.

Their food varies greatly. As a rule, they are frugivorous, but many

cases of carnivorous habit occur. _Scaphander_ has been seen to swallow

_Dentalium_ six at a time, and in six hours the shells of all were

reduced to tiny fragments. _Glaucus_ devours the soft portions of the

pelagic _Porpita_ and _Velella_; _Idalia elegans_ eats its way into

the test of Ascidians, and completely buries itself in the body of its

prey.[403]

The Opisthobranchiata may be classified as follows:--

{ _Bulloidea_

{ 1. TECTIBRANCHIATA { _Aplysioidea_

{ { _Pleurobranchoidea_

{ { _Siphonarioidea_

{

=Opisthobranchiata= { 2. ASCOGLOSSA

{

{ 3. NUDIBRANCHIATA { _Cladohepatica_

{ { _Holohepatica_

{

{ 4. PTEROPODA { _Thecosomata_

{ { _Gymnosomata_

=Sub-order I.= =Tectibranchiata.=--Right ctenidium usually present,

more or less concealed by the mantle fold, visceral ganglia united by a

very long commissure, shell variable in form, more or less enveloped in

folds of the mantle and foot, often becoming rudimentary.

SECTION I. BULLOIDEA.--Shell more or less spiral, internal or external,

epipodia more or less developed, a broad cephalic disc, distinct from

the dorsal region, usually no tentacles, eyes sessile.

FAM. 1. _Actaeonidae._--Shell spiral, solid, entirely covering the

animal; spire generally prominent, operculum corneous, visceral loop

streptoneurous, no epipodia, radula multiseriate, teeth numerous, very

small. Carboniferous ----. Genera: _Actaeon_ (Fig. 286a); _Volvaria_

(Tertiary), _Fortisia_ (Eocene) _Actaeonina_ (Carboniferous),

_Cylindrites_ (Secondary strata), _Actaeonella_ (Cretaceous).

FAM. 2. _Tornatinidae._--Shell spiral, cylindrical, entirely covering

the animal; spire concealed, cephalic disc with two large tentaculiform

appendages behind, no radula. Genera: _Tornatina_ (= _Utriculus_),

_Volvula_.

FAM. 3. _Scaphandridae._--Shell more or less external, covering all or

nearly all the animal, spire concealed, cephalic disc simple or notched

behind, epipodia well developed, radula with first lateral very large,

stomach sometimes with powerful gizzard. Genera: _Scaphander_ (Fig.

287 B); _Sabatia_ (Pliocene), _Smaragdinella_, _Atys_ (Fig. 286 D),

_Cylichna_ (Fig. 286 C), _Amphisphyra_.

FAM. 4. _Bullidae._--Shell external or partly internal, spire quite or

nearly hidden, cephalic disc broad, without appendages, epipodia often

large; radula usually multiseriate. Genera: _Bulla_ (subg. _Haminea_),

_Acera_, mantle with long filiform appendage, epipodia touching over

the shell; _Cylindrobulla_, _Volvatella_.

FAM. 5. _Aplustridae._--Shell partly internal, overlaid by the

posterior part of the cephalic disc, spire not prominent, epipodia

reflected, tentacles auriform. Single genus, _Aplustrum_ (Fig. 286 B;

subg. _Hydatina_).

FAM. 6. _Ringiculidae._--Shell small, solid, covering all the animal;

spire somewhat prominent, aperture narrow, plicated; peristome thick,

sometimes channelled, cephalic disc with a kind of posterior siphon.

Genera: _Ringicula_; _Avellana_ (Cretaceous).

FAM. 7. _Gastropteridae._--Shell completely internal, nautiloid, small;

epipodia very large, rounded, united behind; cephalic disc simple.

Single genus, _Gastropteron_.

FAM. 8. _Philinidae._--Shell completely internal, thin, slightly

spiral; epipodia thick, cephalic disc large, thick, simple; stomach

usually with powerful gizzard. Genera: _Philine_ (Fig. 287 E),

_Colpodaspis_, _Colobocephalus_, _Chelinodura_, _Phanerophthalmus_,

_Cryptophthalmus_.

FAM. 9. _Doridiidae._--Shell completely internal, a mere pellicle with

a small spiral nucleus, mantle with two posterior lobes and a caudal

filament, epipodia reflected. Single genus, _Doridium_.

SECTION II. APLYSIOIDEA.--Shell small, usually not spiral, sometimes

absent, no cephalic disc, head prominent, with two pairs of tentacles,

epipodia large, more or less reflected.

FAM. _Aplysiidae._--Characters those of the section. Genera: _Aplysia_

(Fig. 287 D), shell arched, flattened, animal large (the “sea hare”);

_Dolabella_, shell subtriangular (Fig. 286 F); _Dolabrifer_, shell

sub-quadrangular, not spiral; _Notarchus_, shell microscopic, spiral;

_Phyllaplysia_, body very depressed, oval, no shell.

SECTION III. PLEAUROBRANCHOIDEA.--Dorsal region protected by a wide

_notaeum_ or dorsal covering, or by a shell; no epipodia, ctenidium

large, external, between the right under surface of the notaeum or

shell and the foot; head short, shell present or absent.

FAM. 1. _Pleurobranchidae._--Shell internal or absent, notaeum with

spicules, radula multiseriate. Genera: _Pleurobranchus_ (Fig. 286 H),

(?) _Haliotinella_, _Pleurobranchaea_, (?) _Neda_.

FAM. 2. _Runcinidae._--Branchial lamellae few, under the posterior

right notaeum, no shell. Single genus, _Runcina_.

FAM. 3. _Umbrellidae._--Shell external, depressed patelliform, not

covering all the animal; foot very thick, ctenidium large, head

depressed, small; radula multiseriate, teeth innumerable, very small.

Genera: _Umbrella_ (Fig. 5A, p. 10), _Tylodina_.

SECTION IV. SIPHONARIOIDEA.--Shell patelliform, branchia replaced

wholly or in part by a pulmonary sac, pulmonary orifice closed by a

small lobe, radula multiseriate, teeth very small.

FAM. _Siphonariidae._--Characters those of the section. Genera:

_Siphonaria_ (branchia as well as pulmonary sac), _Gadinia_ (no

branchia). These genera, hitherto placed among the Pulmonata, have been

recently shown (see p. 19) to be modified Opisthobranchiata.

=Sub-order II.= =Ascoglossa.=[404]--Branchia, mantle cavity, and shell

generally wanting, liver ramified, rami enclosed in external papillae

(cerata) or beneath the dorsal surface, kidney not compact, branched;

radula with one series of strong teeth (Fig. 288), worn out teeth at

the front end not dropping off, but preserved in a special sac (ἀσκός).

According to Bergh, the Ascoglossa form a link between the

Tectibranchiata,--especially the Aplysiidae and Bullidae--and the

Cladohepatic Nudibranchs, while the Pleurobranchidae form a somewhat

similar link between the Holohepatic Nudibranchs and the other

Tectibranchiata.

FAM. 1. _Oxynoeidae._[405]--Animal long, tentacles auriform, epipodia

large, simple, or wing-like, a ctenidium and branchial chamber on right

side, shell small, thin, slightly spiral, not covering much of the

body. Genera: _Oxynoe_ (= _Lophocercus_), _Lobiger_.

FAM. 2. _Hermaeidae._--Body depressed, cerata in several rows, no

branchiae, no shell. Genera: _Hermaea_, _Phyllobranchus_, _Stiliger_,

_Alderia_.

FAM. 3. _Elysiidae._--Body depressed, head rather elevated, tentacles

auriform, sides of body dilated into two large wings, which enclose

branches of the liver and sometimes fold over the dorsal surface, no

branchiae, no shell. Genera: _Elysia_, _Thridachia_, _Placobranchus_.

[Illustration: FIG. 288.--Radula of one of Ascoglossa (_Elysia

viridis_ Mont. × 40).]

FAM. 4. _Limapontiidae._--Body slug-like, liver scarcely ramified, no

branchiae, shell, or appendages. Genera: _Limapontia_, _Actaeonia_,

_Cenia_.

=Sub-order III.= =Nudibranchiata.=--Shell absent in the adult, no

ctenidium proper, or osphradium, cerata dorsal or dorso-lateral,

nervous system concentrated, kidney not compact, ramified, penis

retractile, jaws and radula usually present.

SECTION I. CLADOHEPATICA.--Cerata usually latero-dorsal, elongated,

or arborescent, buccal mass strong, jaws present, liver generally

ramified, rami generally entering the cerata.

FAM. 1. _Aeolidiidae._--Body slug-like, head with tentacles and

rhinophores, dorsal area with rows of cerata, which usually contain

sting-cells, radula variable. Genera: _Aeolis_, _Cratena_, _Tergipes_,

_Coryphella_, _Favorinus_, _Facelina_, _Flabellina_, _Fiona_,

_Glaucus_, _Janus_, _Hero_, with many sub-genera.

FAM. 2. _Tethymelibidae._--Body slug-like, large, cerata very large,

no sting-cells, head large, cowl-shaped, no tentacles, rhinophores

much foliated, no radula. Genera: _Tethys_, _Melibe_. The cerata of

_Tethys_, which are capable of independent movement when severed,

have been described as parasitic worms. _Tethys_ feeds on molluscs and

Crustacea.

FAM. 3. _Lomanotidae._--Body slug-like, dorsum prominent, undulating

or lobed, with one row of small cerata, no tentacles, rhinophores

much foliated, radula with uncinated dentate laterals. Single genus,

_Lomanotus_.

FAM. 4. _Dotonidae._--Body slug-like, small, two rows of cerata, each

ceras surrounded by a ring of tubercles, rhinophores simple, radula

uniseriate. Single genus, _Doto_.

FAM. 5. _Dendronotidae._--Body slug-like, somewhat compressed, two rows

of arborescent cerata, no tentacles, frontal margin with arborescent

papillae, rhinophores arborescent, radula multiseriate. Genera:

_Campaspe_, _Dendronotus_.

FAM. 6. _Bornellidae._--Two rows of dorsal papillae, with branchiform

appendages at the base, rhinophores foliate, radula multiseriate.

Single genus, _Bornella_.

FAM. 7. _Scyllaeidae._--Body oblong, compressed, two large foliated

cerata with branchial appendages on the inner side, no tentacles,

rhinophores large, radula multiseriate. Single genus, _Scyllaea_.

FAM. 8. _Phyllirrhoidae._--Body much compressed, with bovine head and

neck, tail tapering, no tentacles, rhinophores simple, teeth few, no

marginals. Single genus, _Phyllirrhoe_.

FAM. 9. _Pleurophyllidiidae._--Body elongate-oval, snout broad, covered

by an arched shield with lateral angles prolonged, branchiae consisting

of two rows of lamellae placed between the notaeum and the foot, no

tentacles, rhinophores short, hidden, radula multiseriate. Single

genus, _Pleurophyllidia_.

FAM. 10. _Pleuroleuridae._--Animal resembling _Pleurophyllidia_, but

without the branchial lamellae. Single genus, _Pleuroleura_.

FAM. 11. _Tritoniidae._--Body long, two rows of unequal arborescent

cerata, rhinophores with ramose appendages, liver not prolonged into

the cerata. Genera: _Tritonia_, _Marionia_.

SECTION 2. HOLOHEPATICA.--Cerata medio-dorsal, retractile or not,

usually paucifoliate, liver never ramified, usually no jaws.

FAM. 1. _Dorididae._--Branchia consisting of a circle or semicircle

of pinnate leaves united at the base, surrounding the anus, almost

always retractile into a cavity, rhinophores foliate, no suctorial

proboscis, radula multiseriate. Genera: _Bathydoris_, _Hexabranchus_,

_Archidoris_ (Fig. 289), _Discodoris_, _Diaulula_, _Cadlina_,

_Centrodoris_, _Platydoris_, _Chromodoris_, _Miamira_, with many

sub-genera.

[Illustration: FIG. 289.--_Doris_ (_Archidoris_) _tuberculata_

L., Britain: _a_, anus; _br_, branchiae surrounding the anus;

_m_, male organ; _rh_, _rh_, rhinophores. × ⅔.]

FAM. 2. _Doriopsidae._--Branchia and rhinophores as in _Dorididae_,

oral aperture pore-shaped, suctorial, no radula. Single genus,

_Doriopsis_.

FAM. 3. _Phyllidiidae._--Body oval, depressed, leathery, a ring of

branchial lamellae, only interrupted by the head and genital papilla,

under the pallial edge, oral aperture pore-shaped, suctorial, no

radula. Genera: _Phyllidia_, _Fryeria_. Bergh unites this and the

preceding family in the group _Porostomata_, which, with Fam. 1, form

the group _Dorididae cryptobranchiatae_.

FAM. 4. _Polyceridae._--Body slug-like, branchiae not retractile,

usually surrounding the anus, rhinophores foliate, tentacles simple,

radula variable, central tooth generally wanting. Genera: _Notodoris_,

_Triopella_, _Aegires_, _Triopa_, _Issa_, _Triopha_, _Crimora_,

_Thecacera_, _Polycerella_, _Palio_, _Polycera_, _Ohola_, _Trevelyana_,

_Nembrotha_, _Euplocamus_, _Plocamopherus_, _Kalinga_.

FAM. 5. _Goniodoridae._--Body oval, depressed, branchia multifoliate,

usually disposed in shape of a horse-shoe, rhinophores foliate,

retractile or not, mouth with a large suctorial proboscis, radula

variable. Genera: _Akiodoris_, _Doridunculus_, _Acanthodoris_,

_Adalaria_, _Lamellidoris_, _Calycidoris_, _Goniodoris_, _Idalia_,

_Ancula_, _Drepania_.

FAM. 6. _Corambidae._--Body otherwise _Doris_-like, but with

two posterior branchiae under the mantle edge, jaws present, no

central tooth, about five laterals. Single genus, _Corambe_ (=

_Hypobranchiaea_). Bergh unites this and the two preceding families in

the group _Dorididae phanerobranchiatae_.

=Sub-order IV.= =Pteropoda.=--The Pteropoda are pelagic animals in

which the lateral portions of the foot are modified into fins, which

are innervated by the pedal ganglia. Their systematic position has

undergone recent revision. It has been the custom to regard them as an

Order of equivalent value to the other four, while some have held them

to be a subdivision of Cephalopoda. Modern authorities, chief among

whom is Pelseneer, regard the Pteropoda not as a primitive, but as a

derived and recent group. They are “Gasteropoda in which the adaptation

to pelagic life has so modified their external characters as to give

them an apparent symmetry.”

The principal point which relates the Pteropoda to the Gasteropoda is

the asymmetry of the visceral organs, intestine, heart, kidney, and

genital gland, which results from their development on one side only

of the body. Their hermaphroditism and the structure of their nervous

system relate them to the Euthyneura rather than to the Streptoneura.

Resemblances in the organs of circulation and generation approximate

them to the Opisthobranchiata rather than to the Pulmonata, while

of the two groups of the former, they tend to closer relationship

with the Tectibranchiata than with the Nudibranchiata. The two

sections of Pteropoda have been considered of distinct origin, the

Thecosomata being derived from the Bulloidea, the Gymnosomata from the

Aplysioidea.[406]

Thus the Pteropoda are a group whose true relations are masked by the

special conditions of their existence, which have tended towards the

development of certain organs, the so-called “wings” and the shell,

which give them an apparent symmetry; this symmetry disappears on a

closer investigation of the internal organs. They are hermaphrodite;

the genital gland has a single efferent duct (except in some

_Cavolinia_), a seminal groove leading to the copulatory organ,

which in the Thecosomata is on the right side of the head, in the

Gymnosomata on the right side of the foot. The genital system resembles

that of the Opisthobranchiata and of the “digonoporous” Pulmonata.

SECTION 1. THECOSOMATA.--Shell or cartilaginoid test always present,

fins united by an intermediate lobe, ctenidia as a rule absent,

replaced by secondary branchiae, no very distinct head or eyes, one

pair of tentacles; cerebral ganglia on the sides of and under the

oesophagus; radula with three rather large teeth in a row, generally

unicuspid, jaw in two pieces, stomach with horny plates, anus generally

on the left side.

The Thecosomata feed on Protozoa and the lower Algae; they have no

proboscis, and the intestine is flexured. The fins are always closely

connected with the head, or what answers to it. About 42 species are

known, belonging to 8 genera.

FAM. 1. _Limacinidae._--Fins very large, branchial chamber dorsal, anus

on right side; shell spiral, sinistral (ultra-dextral, see p. 249),

operculate. Genera: _Limacina_, shell helicoid, deeply umbilicated (_L.

helicina_ swarms in Arctic seas and furnishes food for many Cetacea);

_Peraclis_, spire turreted, aperture large, elongated, produced

anteriorly, no umbilicus; operculum sinistral, in spite of the shell

being ultra-dextral.

FAM. 2. _Cavoliniidae._--Fins large, branchial chamber ventral, shell

a non-spiral cone, angular or round, very thin, embryonic portion

distinct, or formed of two separate plates.

[Illustration: FIG. 290.--Illustrations of Pteropoda Thecosomata:

=A=, _Limacina australis_ Eyd.; =B=, _Cleodora cuspidata_ Bosc.

(shell only); =C=, _Cuvierina columnella_ Rang; =D=, _Creseis

virgula_ Rang; =E=, _Clio balantium_ Rang; _f_, _f_, fins; _l_,

liver; _o_, ovary; _sh_, shell. (After Souleyet.)]

In _Cavolinia_ (= _Hyalaea_, Fig. 5 B, p. 10) the shell consists of two

plates, the ventral being convex, with one to three sharp spines at

the posterior end, the dorsal flatter, without spines. The aperture is

broad, contracted dorso-ventrally. Two long pointed prolongations of

the mantle project from the lateral slits of the shell, and probably

serve to balance the bulky body when swimming. Fins trilobed at the

margin. _Cleodora_ has only rudimentary lateral prolongations, fins

bilobed, shell triangular, angles greatly produced, aperture very

wide, dorsal side keeled. In _Cuvierina_ the shell is straight,

sub-cylindrical, with a median partition, slightly expanding towards

the apex, which is truncated in the adult. The principal sub-genera of

_Clio_ are _Creseis_, which has an elongated sub-cylindrical shell,

sometimes slightly curved, smooth or grooved; and _Clio_ proper, in

which the shell is long, angular, with a dorsal rib, apex (= embryonic

shell) rounded, constricted. _Styliola_ and _Hyalocylix_ also belong to

this group.

FAM. 3. _Cymbuliidae._--Test (which is not homologous with the shell of

other Thecosomata) slipper-shaped, cartilaginoid, simply a thickening

of the mantle; embryo with a calcareous, spiral, operculate shell.

Genera: _Cymbulia_, _Cymbuliopsis_, _Gleba_.

Three other families, Hyalithidae, Pterothecidae, and Conulariidae,

from Palaeozoic strata, are generally added to the Thecosomata. All

are fossil only, and it is doubtful whether they are really Molluscan.

Pelseneer holds that no true fossil Pteropoda occur until the lower

Tertiaries.

SECTION 2. GYMNOSOMATA.--Mantle and shell absent in the adult, fins not

connected by a lobe, no branchial chamber, head well developed, with

two pairs of tentacles, eyes on the posterior pair; cerebral ganglia

above the oesophagus; buccal cavity provided with a pair of protrusible

“hook-sacs,” radula generally with 4 to 12 hooked laterals, central

tooth triangular, jaw in one piece, composed of horny plates, no horny

plates in stomach, anus on the right side.

The Gymnosomata are carnivorous, feeding on Thecosomata and other

pelagic animals, being provided for this purpose with a formidable

buccal armature of hook-sacs and suckers. The intestine, as usual in

carnivorous groups, passes straight from the stomach to the anus; the

fins are not attached to the head, but to the anterior part of the

body. The larva has a straight shell, which disappears in the adult.

About 21 species are known, belonging to 7 genera.

FAM. 1. _Pneumodermatidae._--Animal fusiform, fins rather small, head

prominent, anterior part of buccal cavity protrusible, with suckers on

the ventral side, hook-sacs well marked; branchia on right side, skin

soft, pigmented. Genera: _Dexiobranchaea_, no posterior gill, hook-sacs

short; _Spongiobranchaea_, posterior gill circular; _Pneumoderma_, gill

tetraradiate, hook-sacs long.

FAM. 2. _Clionopsidae._--Body barrel-shaped, proboscis three times the

length of the body, no buccal appendages; hook-sacs short, no lateral

gill, posterior gill tetraradiate, skin not pigmented. _Clionopsis_ is

the single genus.

FAM. 3. _Notobranchaeidae._--Body ovate, buccal appendages conical,

no lateral gill, posterior gill with three radiating crests, skin

pigmented. _Notobranchaea_ is the single genus.

FAM. 4. _Clionidae._--Body long, angulated behind, proboscis short,

mouth with two or three pairs of appendages, no jaw, no gills.

_Clione limacina_ is so abundant in Arctic seas as at times to colour

the surface for miles. Each of the cephalic appendages has about 60,000

minute pedicellated suckers.

[Illustration: FIG. 291.--=A=, Anterior portion of _Pneumoderma_;

=B=, _Clione limacina_ Phipps; =C=, _Halopsyche Gaudichaudi_

Soul.; _f_, _f_, fins; _h.s_, _h.s_, hook-sacs; _l.f_, lobe of

the foot; _s_, _s_, suckers; _o_, posterior genital orifice; _t_,

_t_, tentacles. (After Souleyet.)]

FAM. 5. _Halopsychidae._--Body ovate, thick, rounded behind, no gill

or proboscis, fins long, narrow, broadened at the ends, epidermis

sub-cartilaginoid.

_Halopsyche_ (= _Eurybia_) has the power of withdrawing its head

completely into a sort of pocket, which is closed by an anterior fold

of the mantle. There are two long non-retractile buccal appendages.

=Order IV. Pulmonata=

Gasteropoda with two pairs of tentacles, visceral loop euthyneurous,

ganglia concentrated round the oesophagus; breathing air by a

pallial cavity formed by the union of the front edge of the mantle

with the cervical region, sexes united, shell present or absent, no

operculum[407] (except in _Amphibola_).

=Sub-order I.= =Basommatophora.=--Eyes generally at the base of the

tentacles, which are not retractile, male and female genital orifices

separate, radula (p. 235) multiseriate, shell always present, external.

Fresh water or quasi-marine.

FAM. 1. _Auriculidae._--Breathing organ a pulmonary sac or true lung;

shell spiral, conoidal, internal partitions usually absorbed, aperture

more or less strongly toothed. Jurassic----. Genera: _Auricula_,

_Carychium_, _Scarabus_, _Alexia_, _Tralia_, _Plecotrema_, _Cassidula_,

_Melampus_, _Leuconia_, _Pedipes_ (Fig. 292).

[Illustration: FIG. 292.--Examples of the _Auriculidae_: =A=,

_Auricula Judae_ Lam., Borneo; =B=, _Scarabus Lessoni_ Blainv.,

E. Indies; =C=, _Cassidula mustelina_ Desh., N. Zealand;

=D=, _Melampus castaneus_ Mühlf., S. Pacific; =E=, _Pedipes

quadridens_ Pfr., Jamaica.]

FAM. 2. _Otinidae._--Shell auriform, spire very short. Genera: _Otina_,

_Camptonyx_.--Recent only.

FAM. 3. _Amphibolidae._--A pulmonary sac on right side of neck,

eyes almost pedunculate, shell turbinate, rudely sculptured,

operculate.--Recent. Genus, _Amphibola_ (Fig. 293); subg. _Ampullarina_.

FAM. 4. _Limnaeidae._--Pulmonary sac protected by an external lobe;

shell variable, fragile. Jurassic----. (i.) _Ancylinae_, shell more or

less limpet-shaped. Genera: _Ancylus_, _Gundlachia_, _Latia_. (ii.)

_Limnaeinae_, shell spiral. Genera: _Limnaea_, _Amphipeplea_, _Erinna_,

_Lantzia_, _Pompholyx_, _Choanomphalus_ (with _Carinifex_). (iii.)

_Planorbinae_, shell sinistral, spire flattened or elevated. Genera:

_Planorbis_, _Isidora_ (= _Bulinus_).

FAM. 5. _Physidae._--Mantle more or less reflected over the shell

(radula, Fig. 141C, p. 235); shell sinistral, lustrous. Jurassic----.

Genera: _Physa_, _Aplecta_.

[Illustration: FIG. 293.--_Amphibola avellana_ Chem.]

FAM. 6. _Chilinidae._--Lobe of pulmonary sac large, tentacles broad;

shell ventricose, rather solid; columella plicate. Miocene----. Single

genus, _Chilina_.

=Sub-order II.= =Stylommatophora.=--Two pairs of retractile tentacles

(except in _Janella_), eyes at the tip of the upper pair, male and

female orifices united (except in Vaginulidae and Onchidiidae), no

distinct osphradium.

FAM. 1. _Testacellidae._--Animal carnivorous, slug-like or spirally

coiled, no jaw (whence the name _Agnatha_, often given to this group),

radula with usually few, large, sickle-shaped teeth (p. 232), shell

variable, rarely absent, usually external. Cretaceous----. Principal

genera: _Chlamydephorus_ (shell a simple plate, internal), _Apera_,

_Testacella_ (slug-like, shell terminal), _Strebelia_, _Streptostyla_,

_Glandina_, _Salasiella_, _Petenia_, _Pseudosubulina_, _Streptostele_,

_Tomostele_, _Streptaxis_ (Fig. 203), _Gibbus_, _Ennea_, _Daudebardia_

(Fig. 193), _Schizoglossa_, _Guesteria_, _Aerope_, _Paryphanta_,

_Rhytida_ (subg. _Diplomphalus_, _Elaea_ and _Rhenea_).

[Illustration: FIG. 294.--=A=, _Ennea_ (_Gibbulina_) _palanga_

Fér; =A´= young of same; =B=, Gibbus lyonetianus Pall.]

FAM. 2. _Selenitidae._--Shell internal, external, or absent; jaw

present, radula Testacellidan, central tooth present. Tertiary----.

Genera: _Selenites_, _Plutonia_, _Trigonochlamys_, _Pseudomilax_ (?),

_Rathouisia_ (?).

FAM. 3. _Limacidae._--Shell present or absent, internal or external,

spiral or not, tail often with a mucus pore, jaw (Fig. 107 A, p. 211)

with projecting rostrum on cutting edge, radula with central tooth

tricuspid, laterals bi- or uni-cuspid, marginals aculeate. Eocene----.

Genera: _Otoconcha_, _Urocyclus_, _Mariaella_ (subg. _Tennentia_),

_Parmarion_, _Helicarion_, _Cystopelta_, _Aspidelus_, _Estria_,

_Vitrinopsis_ (subg. _Vitrinoidea_, _Parmella_), _Damayantia_, _Nanina_

(= _Ariophanta_, including _Pachystyla_, _Rhysota_, _Hemiplecta_,

_Trochonanina_, _Euplecta_, _Orpiella_, _Xesta_, _Macrochlamys_,

_Microcystis_, _Sitala_, _Kaliella_, _Durgella_, _Austenia_, _Girasia_,

_Parmacochlea_, _Africarion_, _Sesara_, _Macroceras_, and others),

_Vitriniconus_, _Parmacella_, _Limax_ (subg. _Amalia_ and many

sections), _Vitrina_ (subg. _Vitrinozonites_, _Velifera_), _Zonites_

(subg. _Stenopus_, _Moreletia_, _Mesomphix_, _Hyalinia_, _Gastrodonta_,

_Pristiloma_, _Poecilozonites_, _Thyrophorella_).

FAM. 4. _Philomycidae._--Shell absent, jaw limacidan, radula

helicidan, shield covering all the body. Single genus, _Philomycus_ (=

_Tebennophorus_), with subg. _Pallifera_.

FAM. 5. _Helicidae._--Shell present or absent, internal or external;

jaw of various types, radula with central tooth tricuspid, equal in

size to the first laterals, laterals bi- or tricuspid, marginals

smaller, cusped. Eocene----. Principal genera: _Oopelta_ (no shell),

_Arion_ (shell absent or formed of calcareous granules), _Ariolimax_,

_Geomalacus_, _Anadenus_ (subg. _Prophysaon_), _Hemphillia_,

_Cryptostracon_, _Binneya_, _Helix_ (see below), _Cochlostyla_,

_Bulimus_ (subg. _Borus_, _Orphnus_, _Dryptus_, _Strophochilus_,

_Pachyotus_, and possibly _Caryodes_, _Leucotaenia_, _Liparus_,

_Livinhacea_, _Pachnodus_, _Rachis_, _Atopocochlis_, _Cerastus_,

_Clavator_ belong here, or with _Buliminus_), _Berendtia_, _Rhodea_.

Pilsbry proposes[408] to group _Helix_ as follows:

A. Eggs or young very large at birth:

(1) _Macroön_, incl. _Acavus_, _Pyrochilus_ (= _Phania_), _Stylodonta_,

_Helicophanta_.

B. Eggs or young smaller or minute at birth:

(2) _Belogona._--Female genital system with dart sac and mucus

gland. _Helix_ [restricted] (with sections _Arionta_, _Campylaea_,

_Chilotrema_, _Pomatia_, _Macularia_, _Tachea_, _Iberus_, _Leptaxis_,

_Eulota_, _Fruticicola_, _Xerophila_; _Dorcasia_, _Acusta_,

_Plectotropis_, _Aegista_, _Cathaica_, _Satsuma_, _Euhadra_;

_Lysinoe_), _Gonostoma_, _Leucochroa_, _Allognathus_, _Cochlostyla_,

_Polymita_, _Hemitrochus_ (with sections _Plagioptycha_, _Dialeuca_,

_Coryda_, _Jeanerettia_), _Glyptostoma_, _Acanthinula_, _Vallonia_.

(3) _Teleophalla._--Female system without accessories, male with

flagellum and appendix on penis; no epiphallus. _Sagda_, _Cysticopsis_.

(4) _Epiphallophora._--Female system without accessories, male

with epiphallus on penis; no appendix. _Caracolus_ (with sections

_Lucerna_, _Dentellaria_, _Isomeria_, _Labyrinthus_, _Eurycratera_,

_Parthena_, _Polydontes_, _Theliodomus_, _Cepolis_), _Camaena_ (incl.

_Phoenicobius_), _Obba_, _Chloritis_ (incl. _Hadra_), _Papuina_,

_Planispira_ (subg. _Cristigibba_).

[Illustration: FIG. 295.--Example of the _Macroön_ group of

_Helix_. _Helicophanta Souverbiana_ Fisch., Madagascar, showing

large embryonic shell. × ⅔.]

(5) _Haplogona._--All accessory organs absent, jaw soldered into one

piece. _Polygyra_ (incl. _Daedalochila_, _Triodopsis_, _Mesodon_,

_Stenotrema_), _Endodonta_ (incl. _Libera_, _Charopa_, _Gerontia_,

_Therasia_, and others), _Patula_, _Trochomorpha_, _Anoglypta_.

(6) _Polyplacognatha._--All accessory organs absent, jaw composed

of 16–24 separate plates. _Punctum_, _Laoma_.

[Illustration: FIG. 296.--_Odontostomus pantagruelinus_ Moric.,

S. Brazil. × ½.]

Genera of doubtful position: _Strobilops_, _Ampelita_, _Pedinogyra_,

_Polygyratia_, _Macrocyclis_, _Solaropsis_.

FAM. 6. _Orthalicidae._--Radula, p. 233. Shell external, large,

bulimoid. Single genus, _Orthalicus_; subg. _Liguus_, _Porphyrobaphe_,

_Corona_.

FAM. 7. _Bulimulidae._--Radula, p. 233; jaw, p. 211; shell usually

external. Genera: _Bulimulus_ (incl. _Plecochilus_, _Goniostomus_,

_Drymaeus_, _Liostracus_, _Otostomus_, _Navicula_, _Scutalus_,

_Peronaeus_, _Eurytus_, _Eudioptus_, _Plectostylus_, _Mesembrinus_,

_Mormus_, etc.; _Thaumastus_, _Nesiotis_), _Placostylus_ (incl.

_Charis_), _Amphidromus_, _Partula_, _Calycia_ (?), _Peltella_ (animal

limaciform, shell internal), _Pellicula_, _Amphibulimus_ (incl.

_Simpulopsis_).

FAM. 8. _Cylindrellidae._--Radula, p. 233; shell many whorled, long

turriculate, last whorl often detached, apex often truncated (Fig. 169,

p. 260). Eocene----. Genera: _Cylindrella_ (with sections _Callonia_,

_Thaumasia_), _Leia_, _Macroceramus_, _Pineria_.

FAM. 9. _Pupidae._--Radula, p. 233; shell external, spire usually long,

aperture often narrowed, more or less toothed, often with internal

lamellae. Carboniferous----. Genera: _Anostoma_ (Fig. 154, p. 248),

_Hypselostoma_ (Fig. 202A, p. 302); _Anastomopsis_ (Cretaceous),

_Lychnus_ (Cretaceous), _Boysia_, _Odontostomus_ (incl. _Tomigerus_),

_Buliminus_ (incl. _Petraeus_, _Napaeus_, _Zebrina_, _Mastus_,

_Chondrula_, _Ena_, and perhaps _Rachis_, _Pachnodus_, _Hapalus_,

and others), _Pupa_ (incl. _Torquilla_, _Pupilla_, _Sphyradium_,

_Leucochila_, etc.), _Zospeum_, _Vertigo_, _Megaspira_, _Strophia_,

_Holospira_, _Eucalodium_ (incl. _Coelocentrum_), _Coeliaxis_,

_Perrieria_, _Balea_; _Rillya_ (Eocene), _Clausilia_ (with many

sub-genera), _Rhodina_ (?).

[Illustration: FIG. 297.--=A=, _Clausilia crassicosta_ Ben.,

Sicily; =B=, _Clausilia macarana_ Zieg., Dalmatia; =B´=,

clausilium of same.]

FAM. 10. _Stenogyridae._--Radula, p. 234; shell long, spiral,

shining, more or less translucid, apex blunt, sometimes decollated.

Eocene----. Genera: _Stenogyra_ (subg. _Rumina_, _Obeliscus_, _Opeas_,

_Melaniella_, _Spiraxis_, _Leptinaria_, _Nothus_, _Subulina_,

_Glessula_), _Ferussacia_ (subg. _Cionella_, _Azeca_), _Caecilianella_

(subg. _Geostilbia_). _Achatina_ (shell large, ventricose, columella

strongly truncate), with the sub-genera _Perideris_, _Limicolaria_,

_Columna_, _Pseudachatina_, _Homorus_, probably belongs to a distinct

family.

FAM. 11. _Achatinellidae._--Radula, p. 234; shell small, bulimoid,

indifferently dextral or sinistral. Genera: _Achatinella_ (subg.

_Auriculella_, _Amastra_, _Carelia_), _Tornatellina_.

FAM. 12. _Succineidae._--Radula, p. 234; lower pair of tentacles

wanting or small; shell internal or external, thin, spiral or not, last

whorl large. Eocene----. Genera: _Succinea_, _Homalonyx_, _Hyalimax_,

(?) _Lithotis_, (?) _Catinella_.

FAM. 13. _Janellidae._--Radula, p. 234; animal slug-like, no lower

tentacles, shell an internal plate. Single genus, _Janella_ (=

_Athoracophorus_), with subg. _Aneitea_.

FAM. 14. _Vaginulidae._--Radula, p. 234; animal slug-like, covered with

a coriaceous mantle, lower tentacles bifid, genital orifices widely

separated, male behind the lower right tentacle, female on inferior

median part of right side, anus and pulmonary orifice nearly terminal;

shell absent. Single genus, _Vaginula_ (= _Veronicella_).

[Illustration: FIG. 298.--_Achatina zebra_ Lam., S. Africa. × ½.]

FAM. 15. _Onchidiidae._--Body oval, mantle thick, often warty,

sometimes set with “eyes” (p. 187), two tentacles, genital orifices

widely separate, anus and pulmonary orifice as in _Vaginula_; no shell.

Genera: _Peronia_, _Onchidium_, _Onchidiella_. The family appears to be

an instance of Pulmonata reverting to marine habits of life.

CHAPTER XVI

CLASSES SCAPHOPODA AND PELECYPODA

CLASS SCAPHOPODA

Head rudimentary, mantle edges ventrally concrescent, forming a tube

opening before and behind, and covered with a shell of the same shape;

sexes separate.

[Illustration: FIG. 299.--Anatomy of _Dentalium_: _a_, anterior

aperture of mantle; _f_, foot; _g_, genital gland; _k_, kidney;

_l_, liver. (After Lacaze-Duthiers.)]

The Scaphopoda form a small but very distinct class, whose organisation

is decidedly of a low type. The body is usually slightly curved, the

concave side being the dorsal; muscles near the posterior end attach

the body to the shell. The foot, which can be protruded from the

anterior or wider aperture, is rather long, pointed, and has sometimes

two lateral lobes (_Dentalium_), sometimes a terminal retractile disc

(_Siphonodentalium_), sometimes a retractile disc with a central

tentacle (_Pulsellum_). The cephalic region, as in Pelecypoda, is

covered by the mantle. The mouth is situated on a kind of projection

of the pharynx; the buccal mass, containing the radula (p. 236), is

at the base of the foot, and the intestine branches forward from the

front part of the stomach. The liver (Fig. 299) is paired, and consists

of a number of symmetrical, radiating coeca. There are no eyes, but

on each side of the mouth are small bunches of exsertile filaments

(_captacula_), which appear to act as tactile organs for the seizing

of food. There is no special respiring apparatus, heart or arterial

system, breathing being conducted by the walls of the mantle. The

nervous system has already been described (p. 205).

Two kidneys open on either side of the anus. The genital gland is

large, occupying nearly all the posterior part of the body, the sexual

products being emitted through the right kidney. The veliger has

already been figured (p. 131, Fig. 44). The embryonic shell is formed

of two calcareous laminae, which subsequently unite to form the tube.

With regard to their general relationships, the Scaphopoda resemble

the Gasteropoda in their univalve shell, and in the possession of a

radula; while the pointed foot, the non-lobed velum in the veliger, the

generative system, the bilateral symmetry of the organs generally, and

the absence of any definite head, eyes, or tentacles, are points which

approximate them to the Pelecypoda.

The Scaphopoda are known from Devonian strata to the present time. They

are found at a depth of a few fathoms to very deep water. The only

three genera are _Dentalium_, _Siphonodentalium_ (subg. _Cadulus_), and

_Pulsellum_, which differ in the structure of the foot, as described

above.

CLASS PELECYPODA

Cephalic region rudimentary, mantle consisting of two symmetrical right

and left lobes, covering the body and secreting a bivalve shell hinged

at the dorsal margin; no radula, sexes usually separate. Reference has

already been made to the reproductive system (p. 145), breathing organs

(p. 164 f.), mantle (p. 172), nervous system (p. 205), digestive system

(p. 237 f.), and nomenclature of the various parts of the shell (p. 269

f.).

The shape of the shell, in many Pelecypoda, involving as it does the

position, size, and number of the adductor muscles, is probably due to

mechanical causes, depending on the habits and manner of life of the

individual genus. Thus in a typical dimyarian or two-muscled bivalve,

_e.g._ _Mya_ (Fig. 300, A), the adductor muscles lie well towards each

end of the long axis of the shell, with the hinge about midway between

them. In this position they are best placed for effectually closing the

valves, and since they are nearly equidistant from the axis of motion,

_i.e._ from the hinge, they do an equal amount of work, and are about

equal in size. But in a form like _Modiola_, where the growth of the

shell is irregular in relation to the hinge-line, the anterior muscle

is brought nearer and nearer to the umbones, where its power to do

work, and therefore its size, becomes less and less. But the work to

be done remains the same, and the posterior muscle has to do it nearly

all; hence it moves farther and farther away from the hinge-line, and

at the same time gains in size. In shells like _Ostrea_, _Pecten_,

and _Vulsella_, the anterior muscle, having drawn into line with the

hinge and the posterior muscle, becomes atrophied, while the posterior

muscle, having double work to do, has doubled its size.[409]

[Illustration: FIG. 300.--Illustrating changes in the position

and size of the adductor muscles according to the shape of the

shell: =A=, _Mya_; =B=, _Modiola_; =C=, _Vulsella_. The upper

dotted line shows the hinge-line, the lower connects the two

muscles.]

The development of the foot, again, largely depends upon habits of

life. It is well developed in burrowing forms, while in sessile genera

(_Ostrea_, _Chama_, _Spondylus_) it becomes unnecessary and aborts.

Even in _Pecten_, which does not become sessile, but has ceased to use

the foot as an organ of progression, a similar result follows. Forms

which burrow deeply often “gape” widely, sometimes at one end only,

sometimes at both. _Venus_, _Donax_, _Tellina_, _Mactra_, which are

shallow burrowers, do not gape; _Solen_, _Lutraria_, and to a less

degree _Mya_, burrow deeply and gape widely. In order to burrow deeply

the foot must be highly developed, and the larger it becomes, the

more will it tend to keep the valves apart at the place where it is

habitually protruded. Burrowing species always remain in communication

with the surface by means of their siphons, the constant extension of

which tends to keep the valves apart at the end opposite to the foot.

Burrowing species, again, tend to burrow in such a way as to descend

most easily, and not be impeded by their own shells; in other words,

they act as a wedge, and descend with their narrowest part foremost.

But the burrowing organ, the foot, has to follow suit, and gradually

draws round to the narrowest part of the shell, so that the habitual

deep burrower, such as _Lutraria_, lies with its long axis exactly at

right angles to the surface, its siphons protruding from, and keeping

open, the uppermost or posterior margin of the shell, and the foot

producing the same effect upon the lower or anterior margin. The deeper

the burrower, the more elongated does the shell become, until, through

forms like _Pholas_ and _Saxicava_, we arrive at _Solen_, the most

highly specialised burrower of all, in which the breadth of the shell

is equal throughout, and no obstructive curve exists to impede its

rapid ascent or descent.

The Pelecypoda have been classified in various ways; by the

completeness or sinuation of the pallial line, depending on the absence

or presence of siphons, by the number of adductor muscles, by the

character of the hinge-teeth, and by the number of the branchiae.

For various reasons, none of these methods have proved entirely

satisfactory. That adopted here was suggested by Pelseneer, and

depends upon the character of the branchiae themselves, as suggesting

successive stages of development (p. 166 f.).

=Order I. Protobranchiata=

Branchial filaments not reflected, the two rows inclined at a right

angle (more or less), ventral surface of foot more or less flattened,

byssogenous apparatus little developed, a single anterior aorta,

kidneys distinct, sexes separate, each genital gland opening into the

corresponding kidney.

FAM. 1. _Nuculidae._--Labial palps very large, rows of branchial

filaments at right angles to one another, mantle edges open, siphons

contracted, foot disc-shaped, elongated; shell equivalve, oval, or

produced, interior generally nacreous, hinge with numerous saw-like

teeth. Silurian ----. Principal genera: _Nucula_ (heart dorsal to the

rectum); _Palaeoneilo_ (Devonian), (?) _Sarepta_, _Leda_, _Yoldia_,

_Malletia_; _Tyndaria_ (Upper Tertiary), _Lyrodesma_ (Silurian),

_Actinodonta_ (Silurian), _Babinka_ (Silurian).

FAM. 2. _Solenomyidae._--Labial palps united, one row of branchial

filaments pointing dorsally, the other ventrally; mantle edges in

great part united postero-ventrally, a single siphonal orifice with

two very long tentacles, foot proboscidiform, with a round denticulate

disc at the end; shell equivalve, resembling a Solen, with a strong

corneous periostracum; no hinge-teeth, ligament internal. Single genus,

_Solenomya_. (?) Cretaceous ----].

=Order II. Filibranchiata=

Rows of branchial filaments parallel, pointing ventrally, reflected,

and provided with interfilamentary ciliated junctions, foot usually

with a well-developed byssogenous apparatus.

=Sub-order I.= =Anomiacea.=--Heart dorsal to the rectum, a single

aorta, foot small, anterior adductor very small; shell ostreiform, no

hinge-teeth, fixed by a calcified byssus traversing the right valve

(Fig. 173, p. 262).

FAM. _Anomiidae._--Jurassic ----. Genera: _Anomia_, _Placunanomia_;

_Carolia_ (Eocene), _Placuna_; _Hypotrema_ (Jurassic), _Placunopsis_

(Oolite).

=Sub-order II.= =Arcacea.=--Mantle edge open, both adductors well

developed, heart with two aortae, branchiae free, without interlamellar

junctions, no siphons; renal and generative apertures distinct.

FAM. 1. _Arcadae._--Mantle edge with composite eyes; shell round or

trapezoidal, solid, often with stout bushy periostracum; ligament

often external, on a special area; hinge with numerous lamelliform

teeth. Ordovician ----. Principal genera: _Arca_ (incl. _Barbatia_,

_Scaphula_, and _Cucullaea_), heart dorsal to rectum; _Pectunculus_,

_Glomus_, _Limopsis_; _Trinacria_ and _Nuculina_ (Tertiary).

FAM. 2. _Trigoniidae._--Foot large, hatchet-shaped, with ventral

disc; no byssus, mantle edge with ocelli; shell sub-trigonal,

hinge-teeth few, strong; interior violet-nacreous. Devonian ----.

Genera: _Trigonia_; _Myophoria_ and _Schizodus_ (Trias), _Cyrtonotus_

(Devonian).

[Illustration: FIG. 301.--_Trigonia pectinata_ Lam., Sydney,

N.S.W.]

=Sub-order III.= =Mytilacea.=--Mantle edges fused at one point, anal

orifice distinct, anterior terminal adductor small, one aorta,

branchiae with interfoliary junctions, genital glands penetrating the

side of the mantle and opening by the side of the kidneys.

FAM. _Mytilidae._--Byssus well developed, shell more or less equivalve,

oval, broad; hinge-teeth evanescent. Devonian----. Principal genera:

_Mytilus_?, _Myalina_, _Septifer_, _Modiola_, _Lithodomus_, _Crenella_,

_Dacrydium_, _Myrina_, _Idas_, _Modiolaria_, _Modiolarca_.

=Order III. Pseudolamellibranchiata=

Mantle edges entirely open, foot little developed, anterior adductor

usually aborted, branchial filaments reflected, with interlamellar

junctions, which are sometimes vascular; genital glands opening into

the kidneys or close to the apertures of the kidneys.

FAM. 1. _Aviculidae._--Foot long, tongue-shaped, byssogenous apparatus

well developed, branchiae concrescent with the mantle, adductor muscle

sub-central, at times a small anterior adductor, siphons absent;

shell usually inequivalve, dorsal margin straight, often very long,

winged, lateral teeth much prolonged; structure of shell cellular,

inside prismatic, outside nacreous. Palaeozoic ----. Principal genera:

_Avicula_, including _Meleagrina_, _Malleus_; _Vulsella_ (no wings or

hinge-teeth); _Perna_, including _Crenatula_, _Inoceramus_ (ligaments

in a number of fossettes); _Aucella_ and _Monotis_ (Palaeozoic and

Secondary); _Pterinaea_ and _Ambonychia_ (Palaeozoic); _Pinna_;

_Aviculopinna_ (Carboniferous).

[Illustration: FIG. 302.--_Avicula heteroptera_ Lam., Australia,

showing the inequivalve shell and byssal sinus (_bs_).]

FAM. 2. _Prasinidae._--Shell very small, umbones anterior, incurved,

anterior side depressed, hinge-teeth replaced by dentiform projections

of the lunule fitting into corresponding grooves. Recent. Single genus,

_Prasina_.

FAM. 3. _Ostreidae._--Heart generally ventral to the rectum, branchiae

concrescent with the mantle, no byssus; shell inequivalve, fixed by the

left valve, form irregular. Jurassic ----. Genera: _Ostrea_; _Heligmus_

(Oolite), _Naiadina_ (Cretaceous), _Pernostrea_ (Jurassic).

FAM. 4. _Pectinidae._--Byssus usually absent, mantle edge open,

duplicated, folded back, with pallial ocelli; branchiae not concrescent

with the mantle; shell with unequal “ears” at the umbo, hinge-teeth

lamelliform, often obscure. Silurian----. Principal genera:

_Pedum_, _Chlamys_, _Hinnites_, _Hemipecten_, _Amussium_, _Pecten_;

_Aviculopecten_ (Palaeozoic), _Crenipecten_.

FAM. 5. _Limidae._--Mantle edge as in _Pecten_, tentaculate; shell

sub-equivalve, eared, fixed by a byssus or free. Carboniferous----.

Genera: _Lima_ (Fig. 85, p. 179), _Limea_.

FAM. 6. _Spondylidae._--Foot with a peduncular appendage, no byssus,

numerous pallial ocelli; shell fixed by right valve, surface often

very spinose, two cardinal teeth in each valve. Jurassic----. Genera:

_Plicatula_, _Spondylus_; _Terquemia_ (Lias).

[Illustration: FIG. 303.--_Pecten pallium_ L., East Indies.]

[Illustration: FIG. 304.--_Spondylus petroselinum_ Sowb.,

Mauritius; on a coral.]

FAM. 7. _Dimyidae._--Shell ostreiform, fixed, hinge with or without

symmetrical teeth, two muscular impressions. Single genus, _Dimya_

(Tertiary).

=Order IV. Eulamellibranchiata=

Mantle edges united at one or more points, branchiae with

interfilamentary junctions which are always vascular, genital glands

not opening into the kidneys, usually two adductor muscles.

=Sub-order I. Submytilacea.=--Mantle edges more or less open, anal

orifice distinct, usually no siphons, pallial line usually simple,

cardinal and lateral teeth well marked.

FAM. 1. _Carditidae._--Foot with a byssus or groove, branchiae large,

unequal; shell equivalve, solid, radiately grooved, one or two oblique

cardinal teeth, one or two laterals. Silurian ----. Principal genera:

_Venericardia_, _Cardita_, _Carditella_, _Carditopsis_, _Milneria_;

_Pleurophorus_ (Palaeozoic), _Anodontopsis_ (Silurian).

FAM. 2. _Astartidae._--A short anal siphon, labial palps large;

shell triangular, thick, ligament external, hinge with two or three

cardinals in each valve, laterals obscure.? Devonian ----. Principal

genera: _Astarte_; _Pachytypus_ (Jurassic), _Plesiastarte_ (Eocene),

_Parastarte_, _Woodia_, _Opis_ (Secondary strata), _Prosocoelus_

(Devonian).

FAM. 3. _Crassatellidae._--Mantle with anal orifice or open; shell

equivalve, thick, subtriangular, ligament in an internal fossette,

hinge with two cardinals, laterals produced. Cretaceous----. Principal

genus, _Crassatella_.

FAM. 4. _Cardiniidae._--Shell equivalve, oval or triangular,

ligament external, cardinal teeth small, laterals fairly strong.

Devonian----Oolite. Principal genera: _Cardinia_, _Anthracosia_,

_Carbonicola_, _Anoplophora_.

FAM. 5. _Cyprinidae._--Anal and branchial orifices complete, papillose,

foot thick; shell variable, equivalve, thick, umbones often spiral,

hinge-teeth very variable, ligament external. Jurassic----. Principal

genera: _Cyprina_; _Pygocardia_ (Crag), _Veniella_ (Cretaceous),

_Venilicardia_ (Secondary strata), _Anisocardia_ (Jurassic),

_Isocardia_, _Libitina_, _Coralliophaga_; _Basterotia_ (Eocene).

The families _Pachydomidae_ (Palaeozoic) and _Megalodontidae_

(Palaeozoic--Secondary) are probably related to the Cyprinidae.

[Illustration: FIG. 305.--_Isocardia vulgaris_ Reeve, China.]

FAM. 6. _Aetheriidae._--Anal orifice complete, foot absent, labial

palps large; shell irregular, free or fixed, no hinge-teeth.

Fluviatile, recent only. Genera: _Aetheria_, _Mülleria_, _Bartlettia_.

FAM. 7. _Unionidae._--Foot large and thick, no byssus, anal siphon

short, branchial orifice complete or not, siphon present or absent,

embryo of certain groups passing through a _glochidium_ stage (p. 146);

shell equivalve, sometimes very thick, nacreous within, hinge variable.

Fluviatile. Jurassic----. Principal genera: _Unio_ (subg. _Arconaia_),

_Monocondylaea_, _Pseudodon_, _Anodonta_, _Solenaia_, _Mycetopus_,

_Mutela_, _Spatha_, _Hyria_, _Castalia_, _Leila_.

FAM. 8. _Dreissensiidae._--Both siphons prominent, foot tongue-shaped,

byssiferous; shell mytiliform, with small internal septum. Genera:

_Dreissensia_; _Dreissensiomya_ (Tertiary). The common _Dreissensia

polymorpha_ Pall. was distributed over large parts of Europe in later

Tertiary times. From unknown causes it died out, and has during the

past two hundred years been regaining its position, migrating N. and

W. from its original habitat, the Caspian, by the Volga and its Oka

confluent.

FAM. 9. _Modiolopsidae_.--Shell mytiliform, ligament exterior,

hinge-teeth small, rather numerous. Palaeozoic----. Principal genera:

_Modiolopsis_, _Cyrtodonta_, _Mytilops_, _Ptychodesma_.

FAM. 10. _Lucinidae._--Anal orifice sometimes with a siphon, branchial

orifice complete or not, sometimes a single branchia; foot very long,

vermiform, no byssus, anterior adductor long; shell rounded, equivalve,

blanched, hinge with two cardinals and two laterals in each valve,

sometimes toothless, ligament more or less internal. Silurian----.

Principal genera: _Lucina_, _Corbis_, _Axinus_, _Diplodonta_,

_Montacuta_.

FAM. 11. _Ungulinidae._--Anal orifice complete, foot vermiform, no

byssus, two branchiae; shell equivalve, subcircular, hinge-teeth

variable, no laterals, adductor impressions long, continuing the

pallial line. Tertiary----. Single genus, _Ungulina_.

FAM. 12. _Unicardiidae._--Shell equivalve, round or oval, cardinal

shelf large, a single cardinal in each valve, ligament external.

Carboniferous--Cretaceous. Genera: _Unicardium_, _Scaldia_,

_Pseudedmondia_.

FAM. 13. _Kellyellidae._--Anal siphon prolonged, no marked branchial

orifice; shell very small, oval or round, anterior lateral very strong,

under the cardinal. Eocene----. Genera: _Kellyella_; _Allopagus_ and

_Lutetia_ (Tertiary), _Turtonia_.

FAM. 14. _Erycinidae._--Mantle edges with three apertures, branchial

orifice on the buccal margin, foot long, broadened, with a byssus,

animal usually viviparous. Tertiary----. Genera: _Erycina_, _Kellia_,

_Pythina_, _Lasaea_, _Lepton_.

FAM. 15. _Galeommidae._--Mantle edges more or less reflected

over the shell, apertures and foot as in Erycinidae; shell thin,

equilateral, hinge with few teeth or none. Tertiary----. Genera:

Galeomma, _Scintilla_, _Sportella_, _Chlamydoconcha_, _Hindsiella_,

_Ephippodonta_ (Fig. 32, p. 81).

FAM. 16. _Cyrenidae._--Siphons short, foot large, no byssus; shell

equivalve, subtriangular, with periostracum, hinge with two or three

cardinals, laterals present; animal hermaphrodite, viviparous. Fresh

or brackish water. Jurassic----. Genera: _Cyrena_, _Corbicula_

(subg. _Batissa_, _Velorita_), _Sphaerium_ (= _Cyclas_), _Pisidium_,

_Galatea_, _Fischeria_.

The families _Cyrenellidae_ (single genus, _Cyrenella_) and _Rangiidae_

(single genus, _Rangia_) are probably to be placed here.

=Sub-order II. Tellinacea.=--Siphons long, separate, foot and labial

palps very large, pallial sinus deep, two adductor muscles.

FAM. 1. _Tellinidae._--External branchial fold directed dorsally,

foot with byssogenous slit, but no byssus, branchiae small; shell

compressed, equivalve, ligament external, at least two cardinals in

each valve, laterals variable. Cretaceous----. Principal genera:

_Tellina_ (with many sections), _Gastrana_.

FAM. 2. _Scrobiculariidae._--Animal as in _Tellina_; shell orbiculate

or long oval, equivalve, hinge-teeth weak, ligament in an internal

cavity. Tertiary----. Principal genera: _Scrobicularia_, _Syndosmya_,

_Theora_, _Cumingia_, _Semele_.

FAM. 3. _Donacidae._--External branchial fold directed ventrally; shell

equivalve, subtriangular, solid, smooth, two or three cardinals in each

valve, laterals variable, ligament external. Jurassic----. Genera:

_Donax_, _Iphigenia_, _Isodonta_.

[Illustration: FIG. 306.--_Tellina rastellum_ Hanl., East Indies.]

FAM. 4. _Tancrediidae._--Shell donaciform, ligament external, cardinals

usually two in each valve, posterior laterals strong. Trias----.

Genera: _Tancredia_ (Secondary strata), _Hemidonax_.

FAM. 5. _Cardiliidae._--Shell heart-shaped, hinge as in Mactridae,

posterior adductor resting on a myophore or shelf. Single genus,

_Cardilia_. Tertiary----.

FAM. 6. _Mesodesmatidae._--Mantle edges largely united, with three

orifices, foot byssiferous or not; shell regular or irregular,

usually one cardinal and strong lateral teeth. Tertiary----. Genera:

_Mesodesma_, _Ervilia_.

FAM. 7. _Mactridae._--External branchial fold directed ventrally,

siphons fused, foot tongue-shaped; shell equivalve, triangular-oval,

hinge with ligament in an internal fossette, another portion external,

a bifurcated cardinal tooth in the left valve, fitting into a branching

tooth in the right valve, laterals present. Jurassic----. Genera:

_Nactra_, _Harvella_, _Raëta_, _Eastonia_, _Heterocardia_, _Vanganella_.

=Sub-order III.= =Veneracea.=--Branchiae slightly folded, foot

compressed, siphons generally short, pallial line variable, two

adductor muscles.

FAM. 1. _Veneridae._--Siphons free or partly united, foot seldom

byssiferous; shell solid, equivalve, hinge usually with three cardinal

teeth, laterals variable. Jurassic----. Principal genera: _Cytherea_,

_Circe_; _Grateloupia_ (Tertiary), _Meroe_, _Dosinia_ (= _Artemis_),

_Cyprimeria_, _Cyclina_, _Venus_, _Clementia_, _Lucinopsis_; _Thetis_

(Cretaceous), _Tapes_, _Venerupis_.

FAM. 2. _Petricolidae._--Animal perforating rocks; shell oval, slightly

gaping behind, two or three cardinals, no laterals, pallial sinus well

marked. Recent----. Genera: _Petricola_, _Naranio_.

[Illustration: FIG. 307.--_Cytherea dione_ Lam., Peru.]

FAM. 3. _Glaucomyidae._--Siphons long, united, foot small; shell

produced, thin, hinge with three cardinals, no laterals, pallial sinus

well marked. Recent----. Genus, _Glaucomya_ (incl. _Tanysiphon_).

=Sub-order IV.= =Cardiacea.=--Branchiae much folded back, mantle edges

with three apertures, foot cylindroidal, more or less produced, siphons

present or absent, one or two adductor muscles, pallial line variable.

FAM. 1. _Cardiidae._--Siphons rather long, foot long, no byssus;

shell equivalve, more or less radiately ribbed, hinge with one

or two cardinals in each valve, laterals variable, ligament

external, two adductors. Brackish water or marine. Devonian----.

Genera: _Byssocardium_ and _Lithocardium_ (Tertiary), _Conocardium_

(Palaeozoic), _Cardium_ (with many sections, including _Hemicardium_),

_Limnocardium_ (subg. _Didacna_, _Monodacna_, _Adacna_).

[Illustration: FIG. 308.--_Cardium_ (_Hemicardium_) _cardissa_ L., East

Indies.]

FAM. 2. _Lunulicardiidae._--Shell equivalve, very inequilateral

subtriangular, anterior margin short or truncated, with a deep lunule.

Single genus, _Lunulicardium_ (Palaeozoic).

FAM. 3. _Tridacnidae._--Mantle orifices widely separated, foot short,

byssiferous, no anterior adductor; shell equivalve, large, thick,

usually gaping in front, one cardinal tooth and one or two posterior

laterals in each valve, no pallial sinus. Miocene----. Genera:

_Tridacna_, _Hippopus_. The muscular power of the great _Tridacna_ is

immense. Once caught between their gaping valves, a man’s hand or foot

can scarcely be withdrawn. Two valves of _T. gigas_ in the British

Museum weigh respectively 154 and 156 lbs.

FAM. 4. _Chamidae._--Mantle orifices widely separated, foot short, no

byssus, both adductors present, ovary invading the mantle lobes; shell

fixed, irregularly inequivalve, umbones spiral, ligament external,

cardinal teeth often a mere ridge, anterior lateral strong, nearly

central, no pallial sinus. Jurassic----. Genera: _Chama_; _Diceras_

(Jurassic), attached by one umbo, umbones very prominent, teeth strong;

_Heterodiceras_ (Jurassic), _Requienia_ (Cretaceous), left valve widely

spiral, attached by the umbo, right valve small, fitting on the other

as an operculum, teeth obsolete; _Toucasia_, _Apricardia_, _Matheronia_

(all Secondary strata).

[Illustration: FIG. 309.--=A=, _Requienia ammonea_ Goldf.,

Neocomian, × ½; =B=, _Hippurites cornu-vaccinum_ Goldf.,

Cretaceous, × ¼. _a_, right valve; _f_, point of fixture. (From

Zittel.)]

The four succeeding families require special study in a work on

Palaeontology.

FAM. 5. _Monopleuridae._--Shell very inequivalve, left valve

operculiform, right conical or spiral, fixed at the apex, ligament

prolonged in external grooves. Cretaceous----. Genera: _Monopleura_,

_Valletia_.

FAM. 6. _Caprinidae._--Shell very inequivalve, thick, free or fixed

by apex of right valve, which is spiral or conical, left valve

spiral or not, often perforated by radial canals from the umbo to

the free margin. Neocomian and Cretaceous----. Principal genera:

_Plagioptychus_, _Caprina_, _Ichthyosarcolites_, _Caprotina_,

_Polyconites_.

FAM. 7. _Hippuritidae_ (= _Rudistae_).--Shell very inequivalve,

externally as in Caprinidae, umbo central in left valve, no ligament

proper, left valve with strong hinge-teeth and grooves, two adductor

impressions on prominent myophores, shell structure of the two valves

differing. Cretaceous only. Single genus, _Hippurites_ (Fig. 309, B).

FAM. 8. _Radiolitidae._--Shell inversely conical, biconical, or

cylindrical, general aspect of _Hippurites_, umbo of left valve central

or lateral, right valve with a thick outer layer, often foliaceous,

umbonal cavity partitioned off by laminae. Cretaceous only. Genera:

_Radiolites_, _Biradiolites_.

=Sub-order V.= =Myacea.=--Branchiae much folded back, mantle edges

usually with three openings, foot compressed, siphons large, united or

not, two adductor muscles, pallial line variable.

FAM. 1. _Psammobiidae._--Siphons long, not united, foot large, not

byssiferous; shell equivalve, long, oval, slightly gaping at the ends,

ligament external, prominent, two cardinal teeth in each valve, no

laterals, a deep pallial sinus. Jurassic----. Genera: _Psammobia_,

_Solenotellina_, _Sanguinolaria_, _Asaphis_, _Elizia_, _Quenstedtia_

(Jurassic).

FAM. 2. _Myidae._--Pedal orifice small, siphons long, united in great

part; shell inequivalve, gaping at one or both ends, periostracum

more or less extensive, ligament internal, resting on a prominent

shelf; hinge-teeth variable. Cretaceous----. Genera: _Mya_, _Tugonia_,

_Sphenia_, _Corbula_, _Lutraria_ (for which latter some propose a

separate family).

FAM. 3. _Solenidae._--Foot long, powerful, more or less cylindrical, no

byssus, siphons usually short, united or not, branchiae narrow; shell

equivalve, long and narrow, gaping at both ends, with periostracum,

umbones flattened, ligament external, hinge-teeth variable.?

Devonian----. Genera: _Solecurtus_, _Pharella_, _Pharus_, _Cultellus_,

_Siliqua_, _Ensis_, _Solen_, _Orthonota_ (?), _Palaeosolen_ (?).

FAM. 4. _Glycimeridae._--Pedal orifice very narrow, siphons long,

united in great part, often covered with periostracum; shell more

or less equivalve, gaping at both ends, hinge toothless or with

two weak cardinals, ligament external; animal free or perforating.

Cretaceous----. Genera: _Glycimeris_, _Saxicava_, _Cyrtodaria_.

FAM. 5. _Gastrochaenidae._--Foot small, cylindrical, no byssus,

branchiae narrow, siphons long; shell perforating or cemented to

a shelly tube, gaping widely on the anterior and ventral sides,

no hinge-teeth, a deep pallial sinus. Cretaceous----. Genera:

_Gastrochaena_, _Fistulana_ (tube with a median diaphragm, perforated

by the siphons).

=Sub-order VI.= =Pholadacea.=--Mantle edges largely closed, siphons

long, united, foot short, truncated, disc-shaped, ligament absent, two

adductor muscles; animal perforating.

FAM. 1. _Pholadidae._--Organs contained within the valves, ctenidia

prolonged into the branchial siphon, shell more or less gaping, thin,

dorsal margin in part reflected over the umbones, one or more dorsal

accessory pieces, no hinge-teeth, an interior apophysis proceeding

from the umbonal cavity. Jurassic----. Genera: _Pholas_, _Talona_,

_Pholadidea_ (posterior extremity of the valves prolonged by a corneous

appendage, a passage to the long tube of _Teredo_), _Jouannetia_,

_Xylophaga_, _Martesia_; _Teredina_ (Eocene).

[Illustration: FIG. 310.--_Teredo navalis_ L.: =V=, valves of

shell; =T=, tube; =P=, pallets; =SS=, siphons. (After Möbius.)]

FAM. 2. _Teredinidae._--Animal vermiform, ctenidia mainly within the

branchial siphon, siphons very long, with two calcareous appendages

(“pallets”) near the anterior end, shell very small, continued into a

long calcareous tube, valves deeply notched, internal apophysis as in

Pholadidae. Lias----. Single genus, _Teredo_ (Fig. 310).

=Sub-order VII.= =Anatinacea.=--External branchial fold directed

dorsally, not reflected, sexes united, ovaries and testes with separate

orifices, mantle edges largely united, byssus usually absent, two

adductor muscles, pallial line variable, shell usually nacreous within.

FAM. 1. _Pandoridae._--Siphons short, largely united, foot

tongue-shaped; shell free or fixed, inequivalve, semi-lunar, or

subtriangular, ligament often with calcareous ossicle, pallial line

complete or with slight sinus. Cretaceous----. Genera: _Pandora_,

_Myodora_, _Myochama_.

FAM. 2. _Chamostreidae._--Shell fixed, _Chama_-like, thick, umbones

spiral, ligament with ossicle. Single genus, _Chamostrea_.

FAM. 3. _Verticordiidae._--Siphons not prolonged; shell heart-shaped,

umbones prominent, spiral, ligament with an ossicle, pallial line

complete. Miocene----. Genera: _Verticordia_, _Mytilimeria_,

_Lyonsiella_.

FAM. 4. _Lyonsiidae._--Foot short, byssiferous, siphons short,

separate, shell inequivalve, hinge-teeth usually absent, ligament and

ossicle in an internal groove. Eocene----. Single genus, _Lyonsia_.

[Illustration: FIG. 311.--_Myochama Stutchburyi_ A. Ad., attached

to _Circe undatina_ Lam., Moreton Bay.]

FAM. 5. _Ceromyidae._--Shell inequivalve, large, heart- or

wedge-shaped, hinge toothless, ligament internal in one valve, external

in the other. Secondary strata----. Genera: _Ceromya_, _Gresslya_.

FAM. 6. _Arcomyidae._--Shell equivalve, thin, surface finely

granulated, hinge toothless, cardinal edge dentiform, ligament

external. Secondary and Tertiary strata----. Genera: _Arcomya_,

_Goniomya_, _Pleuromya_, _Machomya_.

FAM. 7. _Anatinidae._--A fourth (?byssal) pallial orifice, siphons

long, separate or fused; shell thin, sometimes inequivalve, exterior

often granulose, ligament often with ossicle, hinge toothless or with

lamellae. Jurassic----. Genera: _Anatina_, _Plectomya_ (Secondary),

_Periploma_, _Cochlodesma_, _Thracia_, _Tyleria_, _Alicia_,

_Asthenothaerus_.

FAM. 8. _Grammysiidae._--Shell equivalve, oval, ligament external,

cardinal margin straight, toothless, pallial line complete.

Palaeozoic----. Principal genus, _Grammysia_, with many other genera of

toothless hinge, but whose exact affinities are uncertain.

FAM. 9. _Praecardiidae._--Shell thin, equivalve or not, radiately

ribbed, margins dentated, sub-umbonal area as in Arca, hinge toothless.

Palaeozoic----. Principal genus, _Praecardium_.

FAM. 10. _Pholadomyidae._--A fourth pallial orifice, siphons very

long, united, foot small; shell thin, equivalve, with radiating ribs,

ligament external, hinge toothless, pallial line sinuate. Jurassic----.

Single genus, _Pholadomya_.

FAM. 11. _Clavagellidae._--Foot rudimentary, siphons long, united,

contained in a long calcareous tube; shell small, one or both

valves soldered in the tube, tube with a centrally fissured disc

at the anterior end, more or less frilled at the posterior end.

Cretaceous----. Genera: _Clavagella_, _Brechites_ (= _Aspergillum_).

=Order V. Septibranchiata=

Mantle edges united at three points, branchiae replaced by a muscular

septum extending from the anterior adductor to the point of separation

of the siphons, septum with symmetrical orifices.

FAM. 1. _Poromyidae._--Branchial septum with groups of lamellae between

the orifices, labial palps large, foot long and narrow, siphons short,

papillose, separated, animal hermaphrodite; shell small, slightly

inequivalve, rounded or oblong, nacreous within. Eocene----. Genera:

_Poromya_, _Silenia_.

FAM. 2. _Cuspidariidae._--Siphons long, united in part, foot short,

animal dioecious; shell small, slightly inequivalve, rostrate, not

nacreous, each valve with ligamentary cartilage spoon-shaped, with

a calcareous ossicle, cardinal and lateral teeth present or absent.

Jurassic----. Single genus, _Cuspidaria_, with many sections.

BRACHIOPODA

PART I

RECENT BRACHIOPODA

BY

ARTHUR E. SHIPLEY, M.A.

Fellow and Tutor of Christ’s College, Cambridge

CHAPTER XVII

RECENT BRACHIOPODA

INTRODUCTION--SHELL--BODY--DIGESTIVE SYSTEM--BODY

CAVITY--CIRCULATORY SYSTEM--EXCRETORY

ORGANS--MUSCLES--NERVOUS SYSTEM--REPRODUCTIVE

SYSTEM--EMBRYOLOGY--HABITS--DISTRIBUTION--CLASSIFICATION

=Introduction=

The group Brachiopoda owes its chief interest to the immense variety

and great antiquity of its fossil forms. Whereas at the present time

the number of extant species amounts to but about 120, Davidson in

his admirable monograph[410] on the British Fossil Brachiopoda has

enumerated close upon 1000 fossil species, found within the limits of

the United Kingdom alone.

The amount of interest that the group in question has excited amongst

naturalists is evinced by the invaluable Bibliography of Brachiopoda,

prepared by the same author and his friend W. H. Dalton.[411] This

monument of patient research contains over 160 quarto pages, each with

the titles of from eighteen to twenty separate papers dealing with

Brachiopods, published between the years 1606 and 1885.

Probably the first reference to Brachiopods in zoological literature

is to be found in a work entitled _Aquatilium et Terrestrium aliquot

Animalium_, published in the year 1606 by Prince Fabio Colonna at Rome.

This work contains the first description of a Brachiopod under the

name of _Concha diphya_. In a second edition, which is not so rare in

our libraries as the first, the author mentions three more species

of Brachiopods. Towards the end of the same century, Martin Lister of

Oxford, in his _Historia sive Synopsis methodica Conchyliorum_ which

appeared in parts, described and figured a considerable number of

Brachiopods, which, under the name of _Anomia_, were until the present

century regarded as Molluscs, and placed in the subdivision Pelecypoda

(Lamellibranchiata).

The first satisfactory figure and description of a _Terebratula_ were

published in the year 1766, in Pallas’ _Miscellanea Zoologica_, still

under the name _Anomia_. In 1781 O. F. Müller figured a _Crania_, under

the name _Patella anomala_, the generic name being subsequently altered

by Cuvier into _Orbicula_.

Bruguière in the year 1789 was the first to recognise the relationship

between _Lingula_ and the other Brachiopods. He for the first time

saw the stalk of this genus, and compared it with that of the stalked

Barnacles, a class of animals which has been more than once associated

with our group.

Cuvier in his _Mémoire sur l’Anatomie de la Lingule_, 1797, gave

the first account of the internal anatomy of a Brachiopod. The same

naturalist first described the nephridia, although his mistake in

considering them lateral hearts was not rectified until the middle of

the present century, when Huxley pointed out that these structures

serve as excretory ducts for the genital products.

Duméril in 1807 proposed the somewhat unfortunate name of Brachiopoda;

and although efforts have been made by de Blainville, who suggested

Palliobranchiata, and more recently by Haeckel, who proposed

Spirobranchiata, to arrive at a name which would be both grammatically

and physiologically more correct, the older name has maintained its

position, and is now universally in use.

In 1834 and 1835 Professor Owen published the results of his researches

into the anatomy of the Brachiopoda. He investigated in these years the

structure of _Waldheimia flavescens_, of a species of _Lingula_ and of

a _Discina_, called by him _Orbicula_. He regarded the group as midway

between the Pelecypoda and the Ascidians. The structure of _Lingula_

was further investigated by Carl Vogt, who in 1851 also supported the

view that the Brachiopoda were related to the Mollusca. But already

in 1847 and 1848 Steenstrup had thrown doubts upon this relationship,

and had maintained that the Order was more closely related to certain

members of the Chaetopoda, a view which afterwards found its ablest

supporter in the American naturalist Morse.

D’Orbigny seems to have been the first observer who drew attention

to the resemblances alleged to exist between the Brachiopoda and the

Polyzoa, and Hancock, in his masterly works _On the Anatomy of the

Fresh-water Bryozoa_ (_Polyzoa_) and in his _Organisation of the

Brachiopoda_, dwelt on these resemblances, and placed the Brachiopoda

between the Polyzoa on the one hand and the Ascidians on the other; a

collocation which subsequently resulted in their inclusion in the now

discarded group of Molluscoidea.

In 1854 Huxley[412] published what is, with the possible exception of

Hancock’s monograph, mentioned above, the most important work upon the

anatomy of the Brachiopoda with which we are acquainted. He corrected

numerous errors of his predecessors and added many new facts to our

knowledge of the group. He was the first to describe the true nature

of the lateral hearts of Cuvier, and to describe the true heart,

afterwards so carefully figured by Hancock.

A further step was made in 1860 and 1861 by the discovery and

description of the larvae of Brachiopoda, by F. Müller and

Lacaze-Duthiers. Since that time we owe what little advance has been

made in the embryology of the group to the researches of Morse and of

Kowalevsky.

Modern methods of research--section cutting, etc.--were first applied

to the group by the Dutch naturalist, van Bemmelen,[413] from whose

admirable historical account of our knowledge of the group many of the

above facts have been gathered. These methods have thrown considerable

light upon the histology of the group, but have not added very much to

our knowledge of the structure or the affinities of the Brachiopoda.

The modern views as to the latter point may be best discussed after

some account of the anatomy of the various genera has been given.

=The Shell=

The body of a Brachiopod is enclosed within a bivalve shell, but the

two halves are not, as they are in the Pelecypoda, one on each side

of the body, but occupy a different position with regard to the main

axes of the body. What this position is, has formed the subject of a

good deal of discussion. For our purpose, however, it will suffice

to distinguish the two valves by the most commonly accepted terms of

_dorsal_ and _ventral_. The former is, as a rule, the smaller

of the two, and usually lies on the lower surface of the animal in

life. Adopting the orientation indicated above, the stalk by means of

which the Brachiopoda are attached to the rocks and stones, etc., upon

which they live, becomes posterior, and the broader edge of the two

shells, which are capable of being opened to some extent, is anterior.

[Illustration: _Fig. 312._--Four specimens of _Terebratulina

caput serpentis_, attached to a waterlogged piece of wood, from

the Clyde area.]

The posterior end of the shell usually narrows, and the ventral valve

projects behind the dorsal, and may be produced into a sort of beak

or funnel, through the aperture of which the stalk protrudes. This

aperture may be completed by the ventral shell, or the latter may only

be notched, in which case the hole is completed by the posterior edge

of the dorsal shell.

The nature of the shell has been used in classifying the group into two

orders:--

I. The Ecardines, whose shell is chitinous but slightly

strengthened by a deposit of calcareous salts. There is no hinge

and no internal supports for the arms. The alimentary canal

terminates in an anus.

II. The Testicardines, whose shells are composed of calcareous

spicules. The valves are hinged together, and there is usually

an internal skeleton supporting the arms. There is no anus.

The outside of the shell of recent Brachiopods is often smooth, but

many are ridged. In a recent species, _Rhynchonella Döderleini_ from

Japan, Davidson[414] has described a number of spines arranged in

concentric circles on the ribbed shell. They are not so long as the

spines irregularly scattered on the shell of _Rh. spinosa_ from the

Jurassic formations. Some shells are brightly coloured, as, for

instance, the various species of _Cistella_ which live on the coralline

rock in the Mediterranean; these exhibit bands or rays of alternate

orange and bright pink. On the other hand, the shells of _Terebratula

vitrea_ are of a slightly translucent white, and of the utmost

delicacy. They are very large, so that the cavity within the valves is

of much greater size than the body of the animal, but in other genera

the soft parts are packed very closely, and there is but a very small

mantle-cavity or space within the shell unoccupied by the body of the

animal. It is, however, more common for the shells of Brachiopods to be

of a dull yellowish colour, and to be somewhat massive. Most species

are attached by a pedicle or stalk to some rock or stone at the bottom

of the sea, but in some, as in _Crania_, the ventral valve becomes

closely adherent to its support, so much so that it is difficult, or

impossible, to remove the animal without leaving the ventral valve

behind. _Lingula_, like _Crania_, one of the Ecardines, lives in sand

(Fig. 321, p. 483), and does not use its long pedicle to adhere to any

fixed object. . The outline of the shell varies extremely. It may be

almost round or prolonged along either axis; the edges of the valves

may be smooth or fluted in correspondence with the ridges and grooves

of the outside of the shell.

[Illustration: FIG. 313.--Three specimens of _Crania anomala_ on

a stone dredged in Loch Fyne. The topmost specimen is seen in

profile.]

On the inner surface of the shell of the Testicardinate Brachiopoda,

at the hinder end of the ventral valve, are two lateral teeth, which

fit into corresponding sockets in the dorsal valve. These form a

hinge, which in many cases is so arranged as to permit the shell

to be opened to only a very limited extent. There are also certain

plate-like processes which project into the lumen of the shell, and

help to support various portions of the body; and in _Terebratula_,

_Waldheimia_, etc., these form a complicated band-like loop, which

increases in complexity with advancing age, and serves to support the

arms. In the extinct _Spirifera_ the internal skeleton takes the form

of two spirally coiled lamellae, which almost entirely fill the cavity

of the shell; the apices of the spirals point outwards (Fig. 330). The

inner surface of the shell also bears the marks of the insertion of the

numerous muscles which govern its movements.

Microscopic examination of thin sections of the shell shows that it

consists of small prisms or spicules of calcareous substance, whose

long axis lies, roughly speaking, at right angles to the surface of the

shell. These spicules are held together by an organic matrix, in which,

however, no cellular elements can be detected. In sections made through

a decalcified shell the position of the spicules which have been

dissolved by the acid is indicated by spaces, and the matrix remains

as a network of fibrils, which end on the outside in a thin cuticular

layer of organic matter. In _Lingula_ and _Discina_ the organic matter

takes a much larger share in the formation of the shell, which in

these genera consists of a number of alternating layers of horny and

calcareous matter. The former is described by Gratiolet as fibrillated,

the fibrils being obliquely placed, whilst the latter consists of a

number of small prisms set at right angles to the surface of the shell.

In many genera, as in _Terebratula_, _Terebratella_, _Cistella_,

_Waldheimia_, _Crania_, etc., the shell is pierced by a number of small

canals (Fig. 314), which in the dried specimens form so many open

pores, but in the living animal contain prolongations of the mantle

or body wall which secretes the shell. They contain extensions of the

layer of epithelial cells which lines and secretes the shell. The

canals come to the surface and at their outer end are often slightly

swollen. They are closed by the cuticular layer which is mentioned

above as covering the shell externally. They are not found in the loops

or other internal processes of the shell. In _Crania_ the canals depart

to some extent from the usual type; instead of running a straight

course to a somewhat swollen outer end, they break up into a number of

very fine branching tubules, which form a very minute meshwork near

the surface of the shell. These fine branches contain protoplasmic

fibrils, which have their origin in the epithelial cells which lie in

the tubules.

By carefully counting the number of tubules in a given area of young

and old specimens of _Waldheimia cranium_, van Bemmelen[415] was able

to show that the spaces between the tubules did not increase with age.

He therefore reasoned that the shells of Brachiopoda do not increase by

intussusception, and that their increase in size must be entirely due

to additions made round their free edge.

The function of the tubules has been a matter of some discussion.

They have been regarded as respiratory organs, but it would seem more

reasonable to suppose that they serve as organs to supply nourishment,

etc., to the organic matrix of the shell.[416]

With the exception of the genus _Crania_, it is usual for Brachiopods

to bear round the edge of their mantle rows or bundles of chitinous

setae or bristles (Figs. 315 and 319). The length and arrangement of

these structures vary in the different species; they are secreted from

little pits in the edge of the mantle. It seems probable that they

serve to some extent as organs of defence, especially in the larva,

where they make their appearance at an early stage; possibly they also

serve as a filter, and prevent the entrance of foreign bodies into

the shell. Their presence has been taken to indicate a certain degree

of affinity between Brachiopods and Chaetopods, since setae are very

characteristic of the last-named group.

=The Body=

The shell of a Brachiopod is secreted partly by the general surface of

the body which is situated at the hinder end of the shell, and partly

by the two leaf-like extensions of the body, which are termed the

dorsal and ventral mantles. These are, in fact, folds of the body wall,

and into them the body cavity and certain of its contents, such as the

liver and generative glands, etc., extend. The space between the two

folds of the mantle, which is limited behind by the anterior wall of

the body, is termed the pallial or mantle cavity. On each side of the

middle line the anterior wall of the body is produced into two “arms,”

which occupy as a rule a considerable part of the mantle cavity. These

arms may be but flattened portions of the general body wall, which

occupies a large part of what in other genera is the mantle of the

dorsal valve, as in _Cistella_ and _Argiope_;[417] or they may be

outgrowths of the body wall in the form of long processes, which are

coiled and twisted in a very characteristic manner in the various

genera. In any case the cross section of the arm shows a groove, one

side of which forms a continuous lip, and the other takes the form of

a single row of tentacles, which are richly ciliated and capable of

considerable movement. The whole arm in _Rhynchonella_ can be protruded

from the shell, as was noted years ago by O. F. Müller, and although

his statement to this effect has often been doubted, its truth was

confirmed by Professor Morse,[418] who writes: “In the year 1872,

while studying living _Rhynchonella_ in the St. Lawrence, I observed a

specimen protrude its arms to a distance of 4 c.m. beyond the anterior

borders of the shell, a distance nearly equalling twice the length of

the shell.” The same observer also mentions that _Lingula_ has the

power of partially protruding its arms. In most genera the cirrhi or

tentacles can alone be protruded.

[Illustration: FIG. 314.--View of the left half of _Cistella_

(_Argiope_) _neapolitana_, which has been cut in two by a median

longitudinal incision, to show the disposition of the organs.

Partly diagrammatic. The inorganic part of the shell only is

shown. The tubular extensions of the mantle and the organic outer

layer are not indicated, and hence the pores appear open.

1. The ventral valve.

2. The dorsal valve.

3. The stalk.

4. The mouth.

5. Lip which overhangs the mouth and runs all round the tentacular

arms.

6. Tentacles.

7. Ovary in dorsal valve.

8. Liver diverticula.

9. Occlusor muscle; its double origin is shown.

10. Internal opening of left nephridium.

11. External opening of left nephridium.

12. Ventral adjuster. The line from 10 crosses the dorsal adjuster.

13. Divaricator muscle.]

The cilia which clothe the tentacles keep up a constant flow of water

into the mantle cavity. This stream not only serves to aerate the blood

of the animals--a process which probably takes place through the thin

inner lining of the mantle--but it also brings with it a number of

diatoms and other minute organisms which serve as food. These particles

become entangled in the tentacles, and are ultimately lodged in the

groove at their base, and passing along this by the action of the cilia

they find their way into the wide mouth, into which the groove deepens

in the posterior median line.

=The Digestive System=

The mouth leads into an oesophagus; this widens into a chamber which

may be termed the stomach (Fig. 314), and which receives the openings

of two large branching glands usually known as the liver. The stomach

passes into a short intestine which is usually bent at about a right

angle with the oesophagus. In the Testicardines the intestine ends

blindly, but in the Ecardines it is of much greater length, and

terminates in an anus, situated posteriorly in the median line in

_Crania_, but asymmetrical and to the right of the body in _Lingula_

(Fig. 315) and _Discina_; in both cases, however, the opening is into

a portion of the mantle cavity. The alimentary canal is supported by

a median dorsal and ventral mesentery, and by two pairs of lateral

mesenteries which pass to the body wall. The lateral mesenteries are

not always quite distinct. When they are, the anterior pair are known

as the gastro-parietal bands, and the posterior as the ileo-parietal.

In _Rhynchonella_ there are two pairs of renal organs, and each of

these mesenteries bears the internal openings of one pair. In all other

Brachiopods there is only one pair, and they are supported by the

ileo-parietal bands.

The alimentary canal is ciliated throughout, and some interesting

observations have been made by Schulgin[419] on the shortening of

these cilia in _Argiope_ (_Cistella_) when the animal is well fed,

and their elongation when the animal is hungry. Amongst the ciliated

cells certain glandular cells have been described. The so-called liver

consists of two more or less branching glands, which open by wide

apertures, one on each side of the stomach. It seems probable that a

good deal of digestion is carried on in these glands, since the diatoms

and other minute organisms upon which the Brachiopoda live are usually

found in the branches of these glands, and the glandular cells lining

the tubules vary much in appearance according to the animal’s state of

nutrition.

=The Body Cavity=

The alimentary canal and liver occupy a considerable portion of the

body cavity or general space of the body; this space is to some extent

cut up by the various mesenteries above mentioned. It also lodges the

reproductive organs and the excretory ducts. Its walls are ciliated,

and the action of the cilia keeps in motion the corpusculated fluid

that bathes the various organs in the body cavity. The mantles, which

are nothing but flattened leaf-like extensions of the body wall lining

the shell, also contain diverticula of the body cavity, which may be

simple flattered spaces or may be broken up into definite channels, as

in _Lingula_ (Fig. 315). It seems not improbable that the body

cavity fluid is aerated through the thin inner layer of the mantle.

[Illustration: FIG. 315.--View of the inner side of a valve

of _Lingula anatifera_ (after François), to show the definite

arrangement of the channels in the mantle: _a_, position of

mouth; _b_, position of anus.]

Running along the base of each arm are two canals, a small one at the

base of the tentacles, which we may term the tentacular canal, and a

larger one, the canal of the lip. The former sends a prolongation into

each tentacle. The latter is, according to Blochmann, a closed canal

in _Crania_, _Lingula_, _Rhynchonella_, and others; but according

to Joubin,[420] it communicates in _Crania_ at one point with the

tentacular canal. It is probably originally a part of the body cavity.

Blochmann[421] states in very definite terms that in _Crania_ neither

the large canal nor the small canal communicates with the general body

cavity, but he admits that in _Lingula_ the small canal opens into that

space.

=The Circulatory System=

The details of the discovery of the central circulatory organ of

Brachiopods form a curious and instructive chapter in the history of

modern morphological inquiry. Hancock, in his monograph on the group,

described and figured on the dorsal surface of the alimentary canal a

well-developed heart, which had been previously noticed by Huxley, who

first showed that the organs which up to his time had been regarded

as hearts were in reality excretory organs. In connexion with this

heart Hancock described numerous arteries, distributed to various

parts of the body. The observers who have written upon the anatomy of

Brachiopods since Hancock’s time, in spite of the fact that they had

at their disposal such refined methods of research as section cutting,

which was quite unknown at the time his monograph was written, have

almost all failed to find this circulatory system, and many of them

have been tempted to deny its existence. Blochmann,[422] however,

in the year 1885 stated that he had found the heart, and had seen

it pulsating in several species of Brachiopoda which he had rapidly

opened whilst alive. Joubin has also described it in large specimens of

_Waldheimia venosa_, and recently Blochmann has published a detailed

account of his work on this subject. Both these authors describe

the heart as a vesicle with muscular walls, situated dorsal to the

alimentary canal. From this, according to Blochmann, a vessel--the

branchio-visceral of Hancock--runs forward as a triangular split in the

dorsal mesentery supporting the alimentary canal. This vessel divides

into two at the oesophagus, and passing through some lacunae in the

walls of this tube, opens into the tentacular canal, and consequently

supplies the tentacles with blood. These two canals which diverge

from the median artery are connected ventrally by a vessel which runs

below the oesophagus; the latter is therefore surrounded by a vascular

ring. Blochmann also describes two pairs of vessels that were seen

and figured by Hancock. A pair of these pass over the gastro-parietal

mesenteries and into the dorsal mantle sinus, the second pair pass

over the ileo-parietal mesenteries and into the ventral mantle sinus;

each of these four arteries runs to one of the four generative

glands, which, as is so usually the case in the animal kingdom, have

thus a specially rich blood supply. If this description should prove

to be correct, the vascular system of Brachiopods shows a striking

resemblance to that of the closed vascular system of the unarmed

Gephyrea, except that the former group has specialised genital vessels.

The blood is colourless.

Joubin’s description of the vascular system of _W. venosa_ differs

in some respects from that of Blochmann. He regards the heart as

collecting the lymph which it receives from numerous lacunar spaces in

the walls of the alimentary canal, and distributing it through various

vessels, which in the main correspond with those of Blochmann, and

which run both to the “arms” and to the generative glands. The latter

vessels, however, open freely into the body cavity, and the fluid which

is forced out from their openings freely bathes the organs found in

the body cavity. Whichever of these accounts should prove to be more

closely in accordance with the facts, there is little doubt that in

addition to the true blood there is a corpusculated fluid in the body

cavity which is to some extent kept in motion by the ciliated cells

that line its walls.

=The Excretory Organs=

The excretory organs (kidneys) which were at one time regarded by

Cuvier and Owen as hearts, are typical nephridia--that is to say,

they are tubes with glandular excretory walls which open at one end

by a wide but flattened funnel-shaped opening into the body cavity,

and at the other end by a circular pore to the exterior (Fig. 314).

In _Rhynchonella_, where there are two pairs of these tubes,--the

only evidence that the group presents of any metameric repetition of

parts,--the inner ends of the anterior pair are supported by the

gastro-parietal mesenteries, and those of the posterior pair by the

ileo-parietal mesenteries. In all other Brachiopods the posterior pair

alone exists. The external opening of these nephridia is near the base

of the anus; in _Cistella_ it is at the bottom of a brood-pouch formed

by the tucking in of the body wall in this neighbourhood, and in this

brood-pouch the eggs develop until the larval stage is reached.

The walls of these nephridia are lined by ciliated cells, amongst

which are some excretory cells, in which numerous brown and yellow

concretions are to be seen; these are probably the nitrogenous excreta

of the animal, and pass out of the body, being washed away by the

stream of water which is constantly passing between the shells.

As in so many other animals, the nephridia act as genital ducts, and

through them the ova and spermatozoa, which break off from the genital

glands and fall into the body cavity, find their way to the outer world.

=The Stalk and Muscles=

The body cavity of a Brachiopod is traversed by several pairs of

muscles, which are very constant in position, and whose contraction

serves to open and close the shell, to move the animal upon its stalk,

and to govern the movements of the arms.

The stalk is absent in _Crania_, and the members of this genus are

attached to the rocks on which they are found by the whole surface of

their ventral valve. In _Lingula_ (Fig. 315) the stalk is long and

hollow, containing what is probably a portion of the body cavity,

surrounded by muscular walls. _Lingula_ is not a fixed form, but lives

half-buried in the sand of the sea-shore (Fig. 321). _Discina_, the

other member of the Ecardines, has a peduncle which pierces the ventral

valve and fixes the animal to its support. Amongst the Testicardines,

_Thecidium_ is also fixed to its supports by the surface of its ventral

valve; the other genera, however, are provided with stalks, which,

being the means of the fixation of the animals, become at the same

time the fixed points upon which their very limited movements can be

effected. The stalk protrudes through the notch or aperture at the

posterior end of the ventral valve, and it probably belongs to the

ventral side of the body. It is in _Cistella_, and doubtless in other

genera, in close organic connexion with both valves, and it seems to

consist of an unusually large development of the supporting tissue

which occurs so frequently in the body of Brachiopods. The surface of

the peduncle is produced into several irregularities and projections

which fit into any depressions of the rock upon which the animal is

fixed.

In _Cistella_ there are four pairs of muscles, two connected with

opening and closing the shell, and two with the movement of the body

upon the stalk (Fig. 314). The most considerable of these muscles are

the two occlusors, which have their origin, one on each side of the

middle line of the dorsal valve, and their insertion by means of a

tendon into the ventral valve. In the species in question each of these

muscles arises by a double head, the two muscles thus formed probably

representing the anterior and posterior occlusors of other forms. The

contraction of these muscles undoubtedly serves to close the shell,

which is opened by a small pair of divaricators arising from the

ventral valve, and inserted into a portion of the dorsal shell which

is posterior to the axis of the hinge. Contraction of these muscles

would thus serve to approximate the posterior edges of the valves and

divaricate the anterior edges and thus to open the shell.

The adjustors are four in number, a ventral pair running from the

ventral valve to be inserted into the stalk, and a corresponding dorsal

pair from the dorsal valve. The simultaneous contraction of either pair

would tend to raise the valve, whilst the alternate contraction of the

muscles of each side would tend to rotate the shell upon the peduncle.

The muscles of _Waldheimia flavescens_ are shown in Fig. 329, and

described briefly on p. 502.

The muscles of the Ecardines differ from those of the Testicardines

inasmuch as they do not terminate in a tendon, but the muscle fibres

run straight from shell to shell. They are also more numerous. In

_Crania_ there is an anterior and a posterior pair of occlusor

muscles, and two pairs of oblique muscles, which seem when they

contract together to move the dorsal shell forwards, or when they

contract alternately to slightly rotate it. In this genus there are

also a pair of protractors and a pair of retractors, and two levators

of the arms, whose function is to draw forward or retract the arms, and

an unpaired median or levator ani muscle. In addition to these bundles

of muscles there are certain muscles in the body wall, and it seems

probable that by their contraction, when the adductors are relaxed,

the body may become somewhat thicker and the valves of the shells will

slightly open.

[Illustration: FIG. 316.--A semi-diagrammatic figure of the

muscular system of _Crania_ (after Blochmann): _a_, anterior

occlusor; _b_, posterior occlusor; _c_, superior oblique; _d_,

inferior oblique; _e_, retractor of the arms; _f_, elevator of

the arms; _g_, protractor of the arms; _h_, unpaired median

muscle. The dorsal valve is uppermost.]

In _Lingula_ (Fig. 322) the muscular system is more complicated;

in addition to the anterior (= anterior laterals) and posterior

(= centrals) pairs of occlusors, there is a single divaricator (=

umbonal), whose contractions in conjunction with those of certain

muscles in the body wall press forward the fluid in the body cavity,

and thus force the valves of the shell apart; and there are three pairs

of adjustor muscles. These latter are called respectively the central

(= middle laterals), external (= external laterals), and posterior

(= transmedians) adjustors, whose action adjusts the shells when all

contract together, and brings about a certain sliding movement of the

shells on one another when they act independently of each other.[423]

=The Nervous System=

The nervous system of Brachiopods is not very clearly understood, and

there are considerable discrepancies in the accounts of the various

investigators, even when they are dealing with the same species. So

much, however, seems certain, that there is a nervous ring surrounding

the oesophagus, that this ring is enlarged dorsally, or, in other

words, near the base of the lip, into a small and inconspicuous dorsal

ganglion, and again ventrally or just behind the base of the tentacles

into a ventral or sub-oesophageal ganglion. The latter is, contrary

to what is usual in Invertebrates, of much larger size than the

supra-oesophageal ganglion, but like the last named, it has retained

its primitive connexion with the ectoderm or outermost layer of the

skin. Both ganglia give off a nerve on each side which runs to the

arms and along the base of the tentacles and lips. The sub-oesophageal

ganglion also gives off nerves which supply the dorsal and ventral

folds of the mantle, the muscles, and other parts.

The modified epithelium in connexion with the ganglia may possibly have

some olfactory or tactile function, but beyond this the Brachiopoda

would appear to be devoid of eyes, ears, or any other kind of sense

organs,--a condition of things doubtless correlated with their sessile

habits, and with the presence of a bivalved shell which leaves no part

of their body exposed.

=The Reproductive System=

The majority of Brachiopods are bisexual, and many authorities regard

the separation of sex as characteristic of the group; on the other

hand, _Lingula pyramidata_ is stated to be hermaphrodite, and it is not

impossible that other species are in the same condition.

The generative organs are of the typical sort, that is, they are formed

from modified mesoblastic cells lining the body cavity. These cells are

heaped up, usually in four places, and form the four ovaries or testes

as the case may be (Fig. 314). The generative glands usually lie partly

in the general body cavity and partly in the dorsal and ventral mantle

folds, two on each side of the body. Along the axis of the heaped-up

cells runs a blood-vessel, which doubtless serves to nourish the

gland, the outer surface of which is bathed in the perivisceral fluid.

Every gradation can be found between the ripe generative cell and the

ordinary cell lining the body cavity. When the ova and spermatozoa

are ripe they fall off from the ovary and testis respectively into

the body cavity, thence they are conveyed to the exterior through the

nephridia. The ova in certain genera, such as _Argiope_, _Cistella_,

and _Thecidium_, develop in brood-pouches which are either lateral

or median involutions of the body wall in the neighbourhood of the

external opening of the nephridia; they are probably fertilised there

by spermatozoa carried from other individuals in the stream of water

which flows into the shell. In other species the ova are thrown out

into the open sea, and their chances of meeting with a spermatozoon is

much increased by the gregarious habits of their sessile parents, for

as a rule considerable numbers of a given species are found in the same

locality.

=The Embryology=

We owe what little we know of the Embryology of the group chiefly to

Kowalevsky,[424] Lacaze-Duthiers,[425] and Morse.[426] The Russian

naturalist worked on _Cistella_ (_Argiope_) _neapolitana_, the French

on _Thecidium_, and the American chiefly on _Terebratulina_.

Although this is not known with any certainty, it seems probable that

the eggs of Brachiopods are fertilised after they have been laid, and

not whilst in the body of the mother. The spermatozoa are doubtless

cast out into the sea by the male, and carried to the female by the

currents set up by the cilia clothing the tentacles.

In _Thecidium_, _Cistella_, and _Argiope_ the first stages of

development, up to the completion of the larva, take place in

brood-pouches; in _Terebratulina_ the eggs pass out of the shell of

the mother and hang in spermaceti-white clusters from her setae and on

surrounding objects. In the course of a few hours they become ciliated

and swim about freely. The brood-pouch in _Thecidium_ is median, in

the convex lower shell, in _Cistella_ it is paired, and arises by the

pushing in of the lateral walls of the body in the region just behind

the horse-shoe-shaped tentacular arms; the renal ducts, which also

serve as oviducts, open into these lateral recesses.

In the female _Thecidium_ (Fig. 317) the two median tentacles which lie

just behind the mouth are enlarged and their ends somewhat swollen;

they are bent back into the brood-pouch, and to them the numerous

larvae are attached by a short filament inserted into the second of the

four segments into which the larva is divided. In _Cistella_ a similar

filament attaches the larvae to the walls of the brood-sac; thus they

are secured from being washed away by the currents constantly flowing

through the mantle cavity of the mother.

[Illustration: FIG. 317.--Brood-pouch of _Thecidium

mediterraneum_. (After Lacaze-Duthiers.) Part of the wall of the

pouch has been removed to show the clusters of larvae.

1. Mouth, overhung by lip.

2. One of the two median tentacles which are enlarged and

modified to bear the larvae.

3. Wall of brood-pouch into which the median tentacles are

folded.

4. Larva attached to the swollen end of the tentacles.]

In _Cistella_ the larva consists at first of two segments, but the

anterior one divides into two, so that in the free swimming larva

we find three segments, the hindermost somewhat longer and narrower

than the others and destined to form the stalk. About the time of

the appearance of the second segment four red eye-spots arise in the

anterior segment, which tends to become constricted off from the

others, and may now be termed the head. It gradually becomes somewhat

umbrella-shaped, develops cilia all over its surface and a special ring

of large cilia round its edge.

In the meantime the second or mantle segment has grown down and

enveloped the stalk, and four bundles of setae have arisen from its

edge. In this stage the larva leaves its mother’s shell and swims

out into the world of water to look for a suitable place on which to

settle down. This is the only stage in the life history of a Brachiopod

when the animal is locomotor, and can serve to spread its species.

The extreme minuteness of the larva and the short time it spends in

this motile condition probably accounts for the fact that Brachiopods

are extremely localised. Where they do occur they are found in great

numbers, rocks being often almost covered with them, but they are not

found over large areas. When viewed under a microscope the larvae seem

to be moving with surprising rapidity, but judging from the analogy of

other forms, it seems doubtful if they swim a yard in an hour.

[Illustration: FIG. 318.--Young larva of _Cistella neapolitana_,

showing three segments, two eye-spots, and two bundles of setae.

(After Kowalevsky.)]

[Illustration: FIG. 319.--Full-grown larva of _Cistella

neapolitana_, with umbrella-shaped head, ciliated. (After

Kowalevsky.)]

Frequently the larva stands on its head for some time, as if

investigating the nature of the rocks on which it may settle; it

is extremely contractile, turning its head from time to time, and

seldom retaining the same outline for any length of time; the setae

are protruded, and at times stick out in every direction; they are

possibly defensive in function. When fully stretched out the larva is

about ⅓ mm. long, but it frequently shortens its body to two-thirds

of this length. The larvae are of a pinkish red colour, with eye-spots

of ruby red. Their colour renders them difficult to discern when they

are swimming over the red coralline rocks upon which they frequently

settle. After swimming about for a few hours the larva fixes itself

finally, apparently adhering by some secretion produced by the stalk

segment. The folds of the second or body segment then turn forward

over the head, and now form the ventral and dorsal mantle folds; these

at once begin to secrete the shell on their outer surfaces. The head

with its eye-spots must be to some extent absorbed, but what goes on

within the mantle is not accurately known. The setae drop off and the

tentacular arms begin to appear as a thickening on the dorsal lobe of

the mantle. They are at first circular in outline. The various changes

which the larva passes through are well illustrated by Morse for

_Terebratulina_, which spawns at Eastport, Me., from April till

August. The different stages are represented in outline in Fig. 320,

taken from his paper.

[Illustration: FIG. 320.--Stages in the development of the larva

of _Terebratulina septentrionalis_. (After Morse.) The youngest

larva has two segments, a third then appears, the larva then

fixes itself, and the second segment folds over the first and

develops bristles round its edge.]

=Habits=

There is little to be said about the habits and natural history of the

Brachiopoda. When once the larva has settled down, the animal never

moves from the spot selected; occasionally it may rotate slightly from

side to side on its stalk, and from time to time it opens its shell. As

so frequently is the case with sessile animals, the sense organs are

reduced to a minimum, the eyes of the larva disappear, and the only

communication which the animal has with the world around it is by

means of the currents set up by the cilia on the tentacles.

In spite of the absence of any definite eyes, _Thecidium_, according to

Lacaze-Duthiers, is sensitive to light; he noticed for instance that,

when his shadow fell across a number of these animals he was watching

in a vessel, their shells, which had been previously gaping, shut up at

once.

In _Cistella_ the tentacles can be protruded from the open shell, and

in _Rhynchonella_ the spirally-coiled arms can be unrolled and extruded

from the shell, but this does not seem to have been observed in other

genera, with the possible exception of _Lingula_. The food of these

animals consists of minute fragments of animal and vegetable matter, a

very large proportion of it being diatoms and other small algae.

[Illustration: FIG. 321.--Figures illustrating the tubes in which

_Lingula anatifera_ lives. The upper figure is a view of the

trilobed opening of the tube. The lower figure shows the tube

in the sand laid open and the animal exposed. The dotted line

indicates the position of the body when retracted. The darker

portion is the tube of sand agglutinated by the secretion of the

stalk. (After François.)]

_Lingula_ differs markedly from the other members of the group,

inasmuch as it is not firmly fixed to a rock or some such body by a

stalk or by one of its valves, but lives in a tube in the sand. Some

recent observations of Mons. P. François[427] on living specimens of

_Lingula anatifera_ which he found living in great numbers on the

sea-shore at Nouméa in New Caledonia may be mentioned. The presence of

the animal is shown by a number of elongated trilobed orifices which

lead into the tube in which the _Lingula_ lives. The animals, like most

other Brachiopods, live well in captivity, and he was able to watch

their habits in the aquaria of his laboratory. The _Lingula_ place

themselves vertically; the anterior end of the body just reaches the

level of the sand; the three lobes into which the orifice of the tube

is divided corresponding with the three brushes of setae which project

from the anterior rim of the mantles. These setae are described by

Morse as projecting in the form of three funnels; currents of water are

seen continually passing in at the side orifices and out through the

central. The tube consists of two portions: an upper part, which is

flattened to correspond with the flat shape of the body, and a lower

part, in which the stalk lies. The upper part is lined with a layer of

mucus, but the sand is not glued together to form a definite tube. The

lower part of the stalk, or the whole when the animal is contracted, is

lodged in a definite tube composed of grains of sand agglutinated by

mucus, probably secreted from the walls of the stalk. At the least sign

of danger the stalk is contracted violently, and the body is withdrawn

to the bottom of the upper portion of the tube. The rapid retreat of

the animal is followed by the collapse of the sand at the mouth of the

tube, and all trace of the presence of the _Lingula_ is lost.