[Illustration: FIG. 20.--Male of _Littorina littoralis_, Lin., removed
from its shell; the mantle-skirt cut along its right line of
attachment and thrown over to the left side of the animal so as to
expose the organs on its inner face.
a, Anus.
i, Intestine.
r, Nephridium (kidney).
r', Aperture of the nephridium.
c, Heart.
br, Ctenidium (gill-plume).
pbr, Parabranchia (= the osphradium or olfactory patch).
x, Glandular lamellae of the inner face of the mantle-skirt.
y, Adrectal (purpuriparous) gland.
t, Testis.
vd, Vas deferens.
p, Penis.
mc, Columella muscle (muscular process grasping the shell).
v, Stomach.
h, Liver.
N.B.--Note the simple snout or rostrum not introverted as a
"proboscis."]
[Illustration: FIG. 21.--Nervous system of _Paludina_ as a type of the
streptoneurous condition. (From Gegenbaur, after Jhering.)
B, Buccal (suboesophageal) ganglion.
C, Cerebral ganglion.
Co, Pleural ganglion.
P, Pedal ganglion with otocyst attached.
p, Pedal nerve.
A, Abdominal ganglion at the extremity of the twisted visceral
"loop."
sp, Supra-intestinal visceral ganglion on the course of the right
visceral cord.
sb, Sub-intestinal ganglion on the course of the left visceral
cord.]
The very large assemblage of forms coming under this order comprises
the most highly developed predaceous sea-snails, numerous vegetarian
species, a considerable number of freshwater and some terrestrial
forms. The partial dissection of a male specimen of the common
periwinkle, _Littorina littoralis_, drawn in fig. 20, will serve to
exhibit the disposition of viscera which prevails in the group. The
branchial chamber formed by the mantle-skirt overhanging the head has
been exposed by cutting along a line extending backward from the
letters vd to the base of the columella muscle mc, and the whole roof
of the chamber thus detached from the right side of the animal's neck
has been thrown over to the left, showing the organs which lie upon
the roof. No opening into the body-cavity has been made; the organs
which lie in the coiled visceral hump show through its transparent
walls. The head is seen in front resting on the foot and carrying a
median non-retractile snout or rostrum, and a pair of cephalic
tentacles at the base of each of which is an eye. In many Gastropoda
the eyes are not thus sessile but raised upon special eye-tentacles
(figs. 25, 56). To the right of the head is seen the muscular penis p,
close to the termination of the vas deferens (spermatic duct) vd. The
testis t occupies a median position in the coiled visceral mass.
Behind the penis on the same side is the hook-like columella muscle, a
development of the retractor muscle of the foot, which clings to the
spiral column or columella of the shell (see fig. 33). This columella
muscle is the same thing as the muscles adhering to the shell in
_Patella_, and the posterior adductor of Lamellibranchs.
The surface of the neck is covered by integument forming the floor of
the branchial cavity. It has not been cut into. Of the organs lying on
the reflected mantle-skirt, that which in the natural state lay
nearest to the vas deferens on the right side of the median line of
the roof of the branchial chamber is the rectum i', ending in the anus
a. It can be traced back to the intestine i near the surface of the
visceral hump, and it is found that the apex of the coil formed by the
hump is occupied by the liver h and the stomach v. Pharynx and
oesophagus are concealed in the head. The enlarged glandular structure
of the walls of the rectum is frequent in the Pectinibranchia, as is
also though not universal the gland marked y, next to the rectum. It
is the adrectal gland, and in the genera _Murex_ and _Purpura_
secretes a colourless liquid which turns purple upon exposure to the
atmosphere, and was used by the ancients as a dye. Near this and less
advanced into the branchial chamber is the single renal organ or
nephridium r with its opening to the exterior r'. Internally this
glandular sac presents a second slit or aperture which leads into the
pericardium (as is now found to be the case in all Mollusca). The
heart c lying in the pericardium is seen in close proximity to the
renal organ, and consists of a single auricle receiving blood from the
gill, and of a single ventricle which pumps it through the body by an
anterior and posterior aorta. The surface x of the mantle between the
rectum and the gill-plume is thrown into folds which in many
sea-snails (whelks or _Buccinidae_, &c.) are very strongly developed.
The whole of this surface appears to be active in the secretion of a
mucous-like substance. The single gill-plume br lies to the left of
the median line in natural position. It corresponds to the right of
the two primitive ctenidia in the untwisted archaic condition of the
molluscan body, and does not project freely into the branchial cavity,
but its axis is attached (by concrescence) to the mantle-skirt (roof
of the branchial chamber). It is rare for the gill-plume of a
Pectinibranch Gastropod to stand out freely as a plume, but
occasionally this more archaic condition is exhibited as in _Valvata_
(fig. 30). Next beyond (to the left of) the gill-plume we find the
so-called parabranchia, which is here simple, but sometimes lamellated
as in _Purpura_ (fig. 22). This organ has, without reason, been
supposed to represent the second ctenidium of the typical mollusc,
which it cannot do on account of its position. It should be to the
right of the anus were this the case. Spengel showed that the
parabranchia of Gastropods is the typical olfactory organ or
osphradium in a highly developed condition. The minute structure of
the epithelium which clothes it, as well as the origin of the nerve
which is distributed to the parabranchia, proves it to be the same
organ which is found universally in molluscs at the base of each
gill-plume, and tests the indrawn current of water by the sense of
smell. The nerve to this organ is given off from the superior
(original right, see fig. 3) visceral ganglion.
[Illustration: FIG. 22.--Female of _Purpura lapillus_ removed from its
shell; the mantle-skirt cut along its left line of attachment and
thrown over to the right side of the animal so as to expose the organs
on its inner face.
a, Anus.
vg, Vagina.
gp, Adrectal purpuriparous gland.
r', Aperture of the nephridium (kidney).
br, Ctenidium (branchial plume).
br', Parabranchia (= the comb-like osphradium or olfactory organ).]
The figures which are given here of various Pectinibranchia are in
most cases sufficiently explained by the references attached to them.
As an excellent general type of the nervous system, attention may be
directed to that of _Paludina_ drawn in fig. 21. On the whole the
ganglia are strongly individualized in the Pectinibranchia, nerve-cell
tissue being concentrated in the ganglia and absent from the cords. At
the same time, the junction of the visceral loop above the intestine
prevents in all Streptoneura the shortening of the visceral loop, and
it is rare to find a fusion of the visceral ganglia with either
pleural, pedal or cerebral--a fusion which can and does take place
where the visceral loop is not above but below the intestine, e.g. in
the Euthyneura (fig. 48), Cephalopoda and Lamellibranchia. As
contrasted with the Aspidobranchia, we find that in the
Pectinibranchia the pedal nerves are distinctly nerves given off from
the pedal ganglia, rather than cord-like nerve-tracts containing both
nerve-cells or ganglionic elements and nerve-fibres. Yet in some
Pectinibranchia (_Paludina_) a ladder-like arrangement of the two
pedal nerves and their lateral branches has been detected. The
histology of the nervous system of Mollusca has yet to be seriously
inquired into.
The alimentary canal of the Pectinibranchia presents little diversity
of character, except in so far as the buccal region is concerned.
Salivary glands are present, and in some carnivorous forms (_Dolium_)
these secrete free sulphuric acid (as much as 2% is present in the
secretion), which assists the animal in boring holes by means of its
rasping tongue through the shells of other molluscs upon which it
preys. A crop-like dilatation of the gut and a recurved intestine,
embedded in the compact yellowish-brown liver, the ducts of which open
into it, form the rest of the digestive tract and occupy a large bulk
of the visceral hump. The buccal region presents a pair of shelly jaws
placed laterally upon the lips, and a wide range of variation in the
form of the denticles of the lingual ribbon or radula.
Well-developed glandular invaginations occur in different positions on
the foot in Pectinibranchia. The most important of these opens by the
ventral pedal pore, situated in the median line in the anterior half
of the foot. This organ is probably homologous with the byssogenous
gland of Lamellibranchs. The aperture, which was formerly supposed to
be an aquiferous pore, leads into an extensive and often ramified
cavity surrounded by glandular tubules. The gland has been found in
both sub-orders of the Pectinibranchia, in _Cyclostoma_ and _Cypraea_
among the Taenioglossa, in _Hemifusus, Cassis, Nassa, Murex,
Fasciolariidae, Turbinellidae, Olividae, Marginellidae_ and _Conidae_
among the Stenoglossa. It was discovered by J.T. Cunningham that in
_Buccinum_ the egg-capsules are formed by this pedal gland and not by
any accessory organ of the generative system. Such horny egg-capsules
doubtless have the same origin in all other species in which they
occur, e.g. _Fusus, Pyrula, Purpura, Murex, Nassa, Trophon, Voluta_,
&c. The float of the pelagic _Janthina_, to which the egg-capsules are
attached, probably is also formed by the secretion of the pedal gland.
[Illustration: FIG. 23.--A, _Triton variegatum_, to show the proboscis
or buccal introvert (e) in a state of eversion.
a, Siphonal notch of the shell occupied by the siphonal fold of the
mantle-skirt (Siphonochlamyda).
b, Edge of the mantle-skirt resting on the shell.
c, Cephalic eye.
d, Cephalic tentacle.
e, Everted buccal introvert (proboscis).
f, Foot.
g, Operculum.
h, Penis.
i, Under surface of the mantle-skirt forming the roof of the
sub-pallial chamber.
B, Sole of the foot of _Pyrula tuba_, to show a, the pore usually
said to be "aquiferous" but probably the orifice of a gland; b,
median line of foot.]
Other glands opening on or near the foot are: (1) The suprapedal gland
opening in the middle line between the snout and the anterior border
of the foot. It is most commonly found in sessile forms and in
terrestrial genera such as _Cyclostoma_; (2) the anterior pedal gland
opening into the anterior groove of the foot, generally present in
aquatic species; (3) dorsal posterior mucous glands in certain
_Cyclostomatidae_.
The foot of the Pectinibranchia, unlike the simple muscular disk of
the Isopleura and Aspidobranchia, is very often divided into lobes, a
fore, middle and hind lobe (pro-, meso- and meta-podium, see figs. 24
and 25). Very usually, but not universally, the metapodium carries an
operculum. The division of the foot into lobes is a simple case of
that much greater elaboration or breaking up into processes and
regions which it undergoes in the class Cephalopoda. Even among some
Gastropoda (viz. the Opisthobranchia) we find the lobation of the foot
still further carried out by the development of lateral lobes, the
parapodia, whilst there are many Pectinibranchia, on the other hand,
in which the foot has a simple oblong form without any trace of lobes.
The development of the Pectinibranchia has been followed in several
examples, e.g. _Paludina, Purpura, Nassa, Vermetus, Neritina_. As in
other Molluscan groups, we find a wide variation in the early process
of the formation of the first embryonic cells, and their arrangement
as a diblastula, dependent on the greater or less amount of food-yolk
which is present in the egg-cell when it commences its embryonic
changes. In fig. 26 the early stages of _Paludina vivipara_ are
represented. There is but very little food-material in the egg of this
Pectinibranch, and consequently the diblastula forms by invagination;
the blastopore or orifice of invagination coincides with the anus, and
never closes entirely. A well-marked trochosphere is formed by the
development of an equatorial ciliated band; and subsequently, by the
disproportionate growth of the lower hemisphere, the trochosphere
becomes a veliger. The primitive shell-sac or shell-gland is well
marked at this stage, and the pharynx is seen as a new ingrowth (the
stomodaeum), about to fuse with and open into the primitively
invaginated arch-enteron (fig. 26, F).
[Illustration: FIG. 24.--Animal and shell of _Phorus exutus_.
a, Snout (not introversible).
b, Cephalic tentacles.
c, Right eye.
d, Pro- and meso-podium; to the right of this is seen the metapodium
bearing the sculptured operculum.]
[Illustration: FIG. 25.--Animal and shell of _Rostellaria
rectirostris_. (From Owen.)
a, Snout or rostrum.
b, Cephalic tentacle.
c, Eye.
d, Propodium and mesopodium.
e, Metapodium.
f, Operculum.
h', Prolonged siphonal notch of the shell occupied by the siphon, or
trough-like process of the mantle-skirt.]
In other Pectinibranchia (and such variations are representative for
all Mollusca, and not characteristic only of Pectinibranchia) we find
that there is a very unequal division of the egg-cell at the
commencement of embryonic development, as in _Nassa_. Consequently
there is, strictly speaking, no invagination (emboly), but an
overgrowth (epiboly) of the smaller cells to enclose the larger. The
general features of this process and of the relation of the blastopore
to mouth and anus have been explained in treating of the development
of Mollusca generally. In such cases the blastopore may entirely
close, and both mouth and anus develop as new ingrowths (stomodaeum
and proctodaeum), whilst, according to the observations of N.
Bobretzky, the closed blastopore may coincide in position with the
mouth in some instances (_Nassa_, &c.), instead of with the anus. But
in these epibolic forms, just as in the embolic _Paludina_, the embryo
proceeds to develop its ciliated band and shell-gland, passing through
the earlier condition of a trochosphere to that of the veliger. In the
veliger stage many Pectinibranchia (_Purpura, Nassa_, &c.) exhibit, in
the dorsal region behind the head, a contractile area of the
body-wall. This acts as a larval heart, but ceases to pulsate after a
time. Similar rhythmically contractile areas are found on the foot of
the embryo Pulmonate _Limax_ and on the yolk-sac (distended
foot-surface) of the Cephalopod _Loligo_. The preconchylian
invagination or shell-gland is formed in the embryo behind the velum,
on the surface opposite the blastopore. It is surrounded by a ridge of
cells which gradually extends over the visceral sac and secretes the
shell. In forms which are naked in the adult state, the shell falls
off soon after the reduction of the velum, but in _Cenia, Runcina_ and
_Vaginula_ the shell-gland and shell are not developed, and the young
animal when hatched has already the naked form of the adult.
[Illustration: FIG. 26.--Development of the River-Snail, _Paludina
vivipara_. (After Lankester, 17.)
dc, Directive corpuscle (outcast cell).
ae, Arch-enteron or cavity lined by the enteric cell-layer or
endoderm.
bl, Blastopore.
vr, Velum or circlet of ciliated cells.
dv, Velar area or cephalic dome.
sm, Site of the as yet unformed mouth.
f, Foot.
mes, Rudiments of the skeleto-trophic tissues.
pi, The pedicle of invagination, the future rectum.
shgl, The primitive shell-sac or shell-gland.
m, Mouth.
an, Anus.
A, Diblastula phase (optical section).
B, The diblastula has become a trochosphere by the development of
the ciliated ring vr (optical section).
C, Side view of the trochosphere with commencing formation of the
foot.
D, Further advanced trochosphere (optical section).
E, The trochosphere passing to the veliger stage, dorsal view
showing the formation of the primitive shell-sac.
F, Side view of the same, showing foot, shell-sac (shgl), velum
(vr), mouth and anus.
N.B.--In this development the blastopore is not elongated; it
persists as the anus. The mouth and stomodaeum form independently
of the blastopore.]
One further feature of the development of the Pectinibranchia deserves
special mention. Many Gastropoda deposit their eggs, after
fertilization, enclosed in capsules; others, as _Paludina_, are
viviparous; others, again, as the Zygobranchia, agree with the
Lamellibranch Conchifera (the bivalves) in having simple exits for the
ova without glandular walls, and therefore discharge their eggs
unenclosed in capsules freely into the sea-water; such unencapsuled
eggs are merely enclosed each in its own delicate chorion. When
egg-capsules are formed they are often of large size, have tough
walls, and in each capsule are several eggs floating in a viscid
fluid. In some cases all the eggs in a capsule develop; in other cases
one egg only in a capsule (_Neritina_), or a small proportion
(_Purpura, Buccinum_), advance in development; the rest are arrested
either after the first process of cell-division (cleavage) or before
that process. The arrested embryos or eggs are then swallowed and
digested by those in the same capsule which have advanced in
development. This is clearly the same process in essence as that of
the formation of a vitellogenous gland from part of the primitive
ovary, or of the feeding of an ovarian egg by the absorption of
neighbouring potential eggs; but here the period at which the
sacrifice of one egg to another takes place is somewhat late. What it
is that determines the arrest of some eggs and the progressive
development of others in the same capsule is at present unknown.
[Illustration: FIG. 27.--_Oxygyrus Keraudrenii_. (From Owen.)
a, Mouth and odontophore.
b, Cephalic tentacles.
c, Eye.
d, Propodium (B) and mesopodium.
e, Metapodium.
f, Operculum.
h, Mantle-chamber.
i, Ctenidium (gill-plume).
k, Retractor muscle of foot.
l, Optic tentacle.
m, Stomach.
n, Dorsal surface overhung by the mantle-skirt; the letter is close
to the salivary gland.
o, Rectum and anus.
p, Liver.
q, Renal organ (nephridium).
s, Ventricle.
u, The otocyst attached to the cerebral ganglion.
w, Testis.
x, Auricle of the heart.
y, Vesicle on genital duct.
z, Penis.]
In the tribe of Pectinibranchia called Heteropoda the foot takes the
form of a swimming organ. The nervous system and sense organs are
highly developed. The odontophore also is remarkably developed, its
lateral teeth being mobile, and it serves as an efficient organ for
attacking the other pelagic forms on which the Heteropoda prey. The
sexes are distinct, as in all Streptoneura; and genital ducts and
accessory glands and pouches are present, as in all Pectinibranchia.
The Heteropoda exhibit a series of modifications in the form and
proportions of the visceral mass and foot, leading from a condition
readily comparable with that of a typical Pectinibranch such as
_Rostellaria_, with the three regions of the foot strongly marked and
a coiled visceral hump of the usual proportions, up to a condition in
which the whole body is of a tapering cylindrical shape, the foot a
plate-like vertical fin, and the visceral hump almost completely
atrophied. Three steps of this modification may be distinguished as
three families:--_Atlantidae, Carinariidae_ and _Pterotrachaeidae_.
They are true Pectinibranchia which have taken to a pelagic life, and
the peculiarities of structure which they exhibit are strictly
adaptations consequent upon their changed mode of life. Such
adaptations are the transparency and colourlessness of the tissues,
and the modifications of the foot, which still shows in _Atlanta_ the
form common in Pectinibranchia (compare fig. 27 and fig. 24). The
cylindrical body of _Pterotrachaea_ is paralleled by the slug-like
forms of Euthyneura. J.W. Spengel has shown that the visceral loop of
the Heteropoda is streptoneurous. Special to the Heteropoda is the
high elaboration of the lingual ribbon, and, as an agreement with some
of the opisthobranchiate Euthyneura, but as a difference from the
Pectinibranchia, we find the otocysts closely attached to the cerebral
ganglia. This is, however, less of a difference than it was at one
time supposed to be, for it has been shown by H. Lacaze-Duthiers, and
also by F. Leydig, that the otocysts of Pectinibranchia even when
lying close upon the pedal ganglion (as in fig. 21) yet receive their
special nerve (which can sometimes be readily isolated) from the
cerebral ganglion (see fig. 11). Accordingly the difference is one of
position of the otocyst and not of its nerve-supply. The Heteropoda
are further remarkable for the high development of their cephalic
eyes, and for the typical character of their osphradium (Spengel's
olfactory organ). This is a groove, the edges of which are raised and
ciliated, lying near the branchial plume in the genera which possess
that organ, whilst in _Firoloida_, which has no branchial plume, the
osphradium occupies a corresponding position. Beneath the ciliated
groove is placed an elongated ganglion (olfactory ganglion) connected
by a nerve to the supra-intestinal (therefore the primitively dextral)
ganglion of the long visceral nerve-loop, the strands of which cross
one another--this being characteristic of Streptoneura (Spengel).
[Illustration: FIG. 28.--_Carinaria mediterranea_. (From Owen.)
A, The animal. B, The shell removed. C, D, Two views of the shell of
_Cardiopoda_.
a, Mouth and odontophore.
b, Cephalic tentacles.
c, Eye.
d, The fin-like mesopodium.
d', Its sucker.
e, Metapodium.
f, Salivary glands.
h, Border of the mantle-flap.
i, Ctenidium (gill-plume).
m, Stomach.
n, Intestine.
o, Anus.
p, Liver.
t, Aorta, springing from the ventricle.
u, Cerebral ganglion.
v, Pleural and pedal ganglion.
w, Testis.
x, Visceral ganglion.
y, Vesicula seminalis.
z, Penis.]
The Heteropoda belong to the "pelagic fauna" occurring near the
surface in the Mediterranean and great oceans in company with the
Pteropoda, the Siphonophorous Hydrozoa, Salpae, Leptocephali, and
other specially-modified transparent swimming representatives of
various groups of the animal kingdom. In development they pass through
the typical trochosphere and veliger stages provided with boat-like
shell.
Sub-order 1.--TAENIOGLOSSA. Radula with a median tooth and three teeth
on each side of it. Formula 3 : 1 : 3.
Tribe 1.--PLATYPODA. Normal Taenioglossa of creeping habit. The foot
is flattened ventrally, at all events in its anterior part
(_Strombidae_). Otocysts situated close to the pedal nerve-centres.
Accessory organs are rarely found on the genital ducts, but occur in
_Paludina, Cyclostoma, Naticidae, Calyptraeidae_, &c. Mandibles
usually present. This is the largest group of Mollusca, including
nearly sixty families, some of which are insufficiently known from the
anatomical point of view.
Fam. 1.--_Paludinidae_. Pedal centres in the form of ganglionated
cords; kidney provided with a ureter; viviparous; fluviatile.
_Paludina_. _Neothauma_, from Lake Tanganyika. _Tylopoma_, extinct,
Tertiary.
[Illustration: FIG. 29.--_Pterotrachea mutica_ seen from the right
side. (After Keferstein.)
a, Pouch for reception of the snout when retracted.
c, Pericardium.
ph, Pharynx.
oc, Cephalic eye.
g, Cerebral ganglion.
g', Pleuro-pedal ganglion.
pr, Foot (mesopodium).
v, Stomach.
i, Intestine.
n, So-called nucleus.
br, Branchial plume (ctenidium).
w, Osphradium.
mt, Foot (metapodium).
z, Caudal appendage.]
Fam. 2.--_Cyclophoridae_. No ctenidium, pallial cavity transformed
into a lung; aperture of shell circular; terrestrial. _Pomatias_,
shell turriculated. _Diplommatina. Hybocystis. Cyclophorus_, shell
umbilicated, with a short spire and horny operculum. Cyclosurus,
shell uncoiled. _Dermatocera_, foot with a horn-shaped protuberance
at its posterior end. Spiraculum.
Fam. 3.--_Ampullariidae_. To the left of the ctenidium a pulmonary
sac, separated from it by an incomplete septum, amphibious.
_Ampullaria_, shell dextral, coiled. _Lanistes_, shell sinistral,
spire short or obsolete. _Meladomus._
Fam. 4.--_Littorinidae._ Oesophageal pouches present; pedal
nerve-centres concentrated; a pedal penis near the right tentacle.
_Littorina_, shell not umbilicated, littoral habit. _Lacuna_, foot
with two posterior appendages, marine, entirely aquatic.
_Cremnoconchus_, entirely aerial, Indian. _Risella. Tectarius._
Fam. 5.--_Fossaridae._ Head with two lobes in some Rhipidoglossa.
_Fossaria._
Fam. 6.--_Purpurinidae_, extinct.
Fam. 7.--_Planaxidae._ Shell with pointed spire; a short pallial
siphon. Planaxis.
Fam. 8.--_Cyclostomatidae._ Pallial cavity transformed into a lung;
pedal centres concentrated; a deep pedal groove. _Cyclostoma_, shell
turbinated, operculum calcareous, British. _Omphalotropis._
Fam. 9.--_Aciculidae._ Pallial cavity transformed into a lung;
operculum horny; shell narrow and elongated. _Acicula._
Fam. 10.--_Valvatidae._ Ctenidium bipectinate, free; hermaphrodite;
fluviatile. _Valvata_, British.
Fam. 11.--_Rissoidae._ Epipodial filaments present; one or two
pallial tentacles. _Rissoa. Rissoina. Stiva._
Fam. 12.--_Litiopidae._ An epipodium bearing three pairs of
tentacles and an operculigerous lobe with two appendages;
inhabitants of the Sargasso weed. _Litiopa._
Fam. 13.--_Adeorbiidae._ Mantle with two posterior appendages;
ctenidium large and capable of protrusion from pallial cavity.
_Adeorbis_, British.
Fam. 14.--_Jeffreysiidae._ Head with two long labial palps; shell
ovoid; operculum horny, semicircular, carinated. _Jeffreysia._
Fam. 15.--_Homalogyridae._ Shell flattened; no cephalic tentacles.
_Homalogyra_, British. _Ammoniceras._
Fam. 16.--_Skeneidae._ Shell depressed, with rounded aperture;
cephalic tentacles long. _Skenea_, British.
Fam. 17.--_Choristidae._ Shell spiral; four cephalic tentacles; eyes
absent; two pedal appendages. _Choristes._
Fam. 18.--_Assimineidae._ Eyes at free extremities of tentacles.
Assiminea, estuarine, British.
Fam. 19.--_Truncatellidae._ Snout very long, bilobed; foot short.
_Truncatella._
[Illustration: FIG. 30.--_Valvata cristata_, Müll.
o, Mouth.
op, Operculum.
br, Ctenidium (branchial plume).
x, Filiform appendage (? rudimentary ctenidium).
The freely projecting ctenidium of typical form not having its axis
fused to the roof of the branchial chamber is the notable
character of this genus.]
Fam. 20.--_Hydrobiidae._ Shell with prominent spire; penis distant
from right tentacle, generally appendiculated; brackish water or
fluviatile. _Hydrobia_, British. _Baikalia_, from Lake Baikal.
_Pomatiopsis. Bithynella. Lithoglyphus. Spekia_, viviparous, from
Lake Tanganyika. _Tanganyicia. Limnotrochus_, from Lake Tanganyika.
_Chytra. Littorinida. Bithynia_, British, fluviatile. _Stenothyra._
Fam. 21.--_Melaniidae._ Spire of shell somewhat elongated;
mantle-border fringed; viviparous; fluviatile. _Melania. Faunus.
Paludomus. Melanopsis. Nassopsis. Bythoceras_, from Lake
Tanganyika.
Fam. 22.--_Typhobiidae._ Foot wide; shell turriculated, with
carinated whorls, the carinae tuberculated or spiny. _Typhobia.
Bathanalia_, from Lake Tanganyika.
Fam. 23.--_Pleuroceridae._ Like _Melaniidae_, but mantle-border not
fringed and reproduction oviparous. _Pleurocera. Anculotus._
Fam. 24.--_Pseudomelaniidae._ All extinct.
Fam. 25.--_Subulitidae._ All extinct.
Fam. 26.--_Nerineidae._ All extinct.
Fam. 27.--_Cerithiidae._ Shell with numerous tuberculated whorls;
aperture canaliculated anteriorly; short pallial siphon. _Cerithium.
Bittium. Potamides. Triforis. Laeocochlis. Cerithiopsis._
Fam. 28.--_Modulidae._ Shell with short spire; no siphon.
_Modulus._
Fam. 29.--_Vermetidae._ Animal fixed by the shell, the last whorls
of which are not in contact with each other; foot small; two
anterior pedal tentacles. _Vermetus. Siliquaria._
Fam. 30.--_Caecidae._ Shell almost completely uncoiled, in one
plane, with internal septa. _Caecum_, British.
Fam. 31.--_Turritellidae._ Shell very long; head large; foot broad.
_Turritella_, British. _Mesalia. Mathilda._
Fam. 32.--_Struthiolariidae._ Shell conical; aperture slightly
canaliculated; siphon slightly developed. _Struthiolaria._
Fam. 33.--_Chenopodidae._ Shell elongated; aperture expanded; siphon
very short. _Chenopus_, British. _Alaria, Spinigera, Diartema_,
extinct.
Fam. 34.--_Strombidae._ Foot narrow, compressed, without sole.
_Strombus. Pteroceras. Rostellaria. Terebellum._
Fam. 35.--_Xenophoridae._ Foot transversely divided into two parts.
_Xenophorus. Eotrochus_, Silurian.
Fam. 36.--_Capulidae._ Shell conical, not coiled, but slightly
incurved posteriorly; a tongue-shaped projection between snout and
foot. _Capulus. Thyca_, parasitic on asterids. _Platyceras_,
extinct.
[Illustration: FIG. 31.--Shell of _Crucibulum_, seen from below so as
to show the inner whorl b, concealed by the cap-like outer whorl a.]
Fam. 37.--_Hipponycidae._ Shell conical; foot secreting a ventral
calcareous plate; animal fixed. _Hipponyx. Mitrularia._
Fam. 38.--_Calyptraeidae._ Shell with short spire; lateral cervical
lobes present; accessory genital glands. _Calyptraea_, British.
_Crepidula. Crucibulum._
Fam. 39.--_Naricidae._ Foot divided into two, posterior half bearing
the operculum; a wide epipodial velum; shell turbinated. Narica.
Fam. 40.--_Naticidae._ Foot large, with aquiferous system; propodium
reflected over head; eyes degenerate; burrowing habit. _Natica_,
British. _Amaura. Sigaretus._
Fam. 41.--_Lamellariidae._ Shell thin, more or less covered by the
mantle; no operculum. _Lamellaria. Velutina. Marsenina_,
_Oncidiopsis_, hermaphrodite.
Fam. 42.--_Trichotropidae._ Shell with short spire, carinate and
pointed. _Trichotropis._
Fam. 43.--_Seguenziidae._ Shell trochiform, with canaliculated
aperture and twisted columella. _Seguenzia_, abyssal.
Fam. 44.--_Janthinidae._ Shell thin; operculum absent; tentacles
bifid; foot secretes a float; pelagic. _Janthina. Recluzia._
Fam. 45.--_Cypraeidae._ Shell inrolled, solid, polished, aperture
very narrow in adult; short siphon; anus posterior; osphradium with
three lobes; mantle reflected over shell. _Cypraea. Pustularia.
Ovula. Pedicularia_, attached to corals. _Erato_.
[Illustration: FIG. 32.--Animal and shell of _Ovula_.
b, Cephalic tentacles.
d, Foot.
h, Mantle-skirt, which is naturally carried in a reflected condition
so as to cover the sides of the shell.]
Fam. 46.--_Tritonidae._ Shell turriculated and siphonated, thick,
each whorl with varices; foot broad and truncated anteriorly;
pallial siphon well developed; proboscis present. _Triton. Persona._
_Ranella._
Fam. 47.--_Columbellinidae._ All extinct.
Fam. 48.--_Cassididae._ Shell ventricose, with elongated aperture,
and short spire; proboscis and siphon long; operculum with marginal
nucleus. _Cassis. Cassidaria. Oniscia._
Fam. 49--_Oocorythidae._ Shell globular and ventricose; aperture
oval and canaliculated; operculum spiral. _Oocorys_, abyssal.
Fam. 50.--_Doliidae._ Shell ventricose, with short spire, and wide
aperture; no varices and no operculum; foot very broad, with
projecting anterior angles; siphon long. _Dolium. Pyrula._