In this division the torsion of the visceral mass and visceral
commissure is at its maximum, the latter being twisted into a figure of
eight. The right half of the commissure with its ganglion is
supra-intestinal, the left half with its ganglion infra-intestinal. In
some cases each pleural ganglion is connected with the opposite branch
of the visceral commissure by anastomosis with the pallial nerve, a
condition which is called dialyneury; or there may be a direct
connective from the pleural ganglion to the visceral ganglion of the
opposite side, which is called zygoneury. The head bears only one pair
of tentacles. The radular teeth are of several different kinds in each
transverse row. The heart is usually posterior to the branchia
(proso-branchiate). The sexes are usually separate.
The old division into Zygobranchia and Azygobranchia must be abandoned,
for the Azygobranchiate Rhipidoglossa have much greater affinity to the
Zygobranchiate _Haliotidae_ and _Fissurellidae_ than to the
Azygobranchia in general. This is shown by the labial commissure and
pedal cords of the nervous system, by the opening of the gonad into the
right kidney, and by other points. Further, the _Pleurotomariidae_ have
been discovered to possess two branchiae. The sub-class is now divided
into two orders: the Aspidobranchia in which the branchia or ctenidium
is bipectinate and attached only at its base, and the Pectinibranchia in
which the ctenidium is monopectinate and attached to the mantle
throughout its length.
[Illustration: FIG. 4.--The Common Limpet (_Patella vulgata_) in its
shell, seen from the pedal surface. (Lankester.)
x, y, The median antero-posterior axis.
a, Cephalic tentacle.
b, Plantar surface of the foot.
c, Free edge of the shell.
d, The branchial efferent vessel carrying aerated blood to the
auricle, and here interrupting the circlet of gill lamellae.
e, Margin of the mantle-skirt.
f, Gill lamellae (_not_ ctenidia, but special pallial growths,
comparable with those of Pleurophyllidia).
g, The branchial efferent vessel.
h, Factor of the branchial advehent vessel.
i, Interspaces between the muscular bundles of the root of the foot,
causing the separate areae seen in fig. 5, c.]
Order I. ASPIDOBRANCHIA.--These are the most primitive Gastropods,
retaining to a great degree the original symmetry of the organs of the
pallial complex, having two kidneys, in some cases two branchiae, and
two auricles. The gonad has no accessory organs and except in
_Neritidae_ no duct, but discharges into the right kidney.
[Illustration: FIG. 5.--Dorsal surface of the Limpet removed from its
shell and deprived of its black pigmented epithelium; the internal
organs are seen through the transparent body-wall. (Lankester.)
c, Muscular bundles forming the root of the foot, and adherent to
the shell.
e, Free mantle-skirt.
em, Tentaculiferous margin of the same.
i, Smaller (left) nephridium.
k, Larger (right) nephridium.
l, Pericardium.
lx, Fibrous septum, behind the pericardium.
n, Liver.
int, Intestine.
ecr, Anterior area of the mantle-skirt over-hanging the head
(cephalic hood).]
Forms adapted to terrestrial life and to aerial respiration occur in
various divisions of Gastropods, and do not constitute a single
homogeneous group. Thus the _Helicinidae_, which are terrestrial, are
now placed among the Aspidobranchia. In these there are neither
branchia nor osphradium, and the pallial chamber which retains its
large opening serves as a lung. Degeneration of the shell occurs in
some members of the order. It is largely covered by the mantle in some
_Fissurellidae_, is entirely internal in _Pupilia_ and absent in
_Titiscaniidae_.
The common limpet is a specially interesting and abundant example of
the more primitive Aspidobranchia. The foot of the limpet is a nearly
circular disk of muscular tissue; in front, projecting from and raised
above it, are the head and neck (figs. 4, 13). The visceral hump forms
a low conical dome above the sub-circular foot, and standing out all
round the base of this dome so as completely to overlap the head and
foot, is the circular mantle-skirt. The depth of free mantle-skirt is
greatest in front, where the head and neck are covered in by it. Upon
the surface of the visceral dome, and extending to the edge of the
free mantle-skirt, is the conical shell. When the shell is taken away
(best effected by immersion in hot water) the surface of the visceral
dome is found to be covered by a black-coloured epithelium, which may
be removed, enabling the observer to note the position of some organs
lying below the transparent integument (fig. 5). The muscular columns
(c) attaching the foot to the shell form a ring incomplete in front,
external to which is the free mantle-skirt. The limits of the large
area formed by the flap over the head and neck (ecr) can be traced,
and we note the anal papilla showing through and opening on the right
shoulder, so to speak, of the animal into the large anterior region of
the sub-pallial space. Close to this the small renal organ (i, mediad)
and the larger renal organ (k, to the right and posteriorly) are seen,
also the pericardium (l) and a coil of the intestine (int) embedded
in the compact liver.
[Illustration: Fig. 6.--Anterior portion of the same Limpet, with the
overhanging cephalic hood removed. (Lankester.)
a, Cephalic tentacle.
b, Foot.
c, Muscular substance forming the root of the foot.
d, The capito-pedal organs of Lankester (= rudimentary ctenidia).
e, Mantle-skirt.
f, Papilla of the larger nephridium.
g, Anus.
h, Papilla of the smaller nephridium.
i, Smaller nephridium.
k, Larger nephridium.
l, Pericardium.
m, Cut edge of the mantle-skirt.
n, Liver.
p, Snout.]
On cutting away the anterior part of the mantle-skirt so as to expose
the sub-pallial chamber in the region of the neck, we find the right
and left renal papillae (discovered by Lankester in 1867) on either
side of the anal papilla (fig. 6), but no gills. If a similar
examination be made of the allied genus _Fissurella_ (fig. 17, d), we
find right and left of the two renal apertures a right and left
gill-plume or ctenidium, which here as in _Haliotis_ and
_Pleurotomaria_ retain their original paired condition. In _Patella_
no such plumes exist, but right and left of the neck are seen a pair
of minute oblong yellow bodies (fig. 6, d), which were originally
described by Lankester as orifices possibly connected with the
evacuation of the generative products. On account of their position
they were termed by him the "capito-pedal orifices," being placed near
the junction of head and foot. J.W. Spengel has, however, in a most
ingenious way shown that these bodies are the representatives of the
typical pair of ctenidia, here reduced to a mere rudiment. Near to
each rudimentary ctenidium Spengel has discovered an olfactory patch
or osphradium (consisting of modified epithelium) and an olfactory
nerve-ganglion (fig. 8). It will be remembered that, according to
Spengel, the osphradium of mollusca is definitely and intimately
related to the gill-plume or ctenidium, being always placed near the
base of that organ; further, Spengel has shown that the nerve-supply
of this olfactory organ is always derived from the visceral loop.
Accordingly, the nerve-supply affords a means of testing the
conclusion that we have in Lankester's capito-pedal bodies the
rudimentary ctenidia. The accompanying diagrams (figs. 9, 10) of the
nervous systems of _Patella_ and of _Haliotis_, as determined by
Spengel, show the identity in the origin of the nerves passing from
the visceral loop to Spengel's olfactory ganglion of the Limpet, and
that of the nerves which pass from the visceral loop of _Haliotis_ to
the olfactory patch or osphradium, which lies in immediate relation on
the right and on the left side to the right and left gill-plumes
(ctenidia) respectively. The same diagrams serve to demonstrate the
streptoneurous condition of the visceral loop in Aspidobranchia.
[Illustration: FIG. 7.--The same specimen viewed from the left front,
so as to show the sub-anal tract (ff) of the larger nephridium, by
which it communicates with the pericardium. o, Mouth; other letters as
in fig. 6.]
Thus, then, we find that the limpet possesses a symmetrically disposed
pair of ctenidia in a rudimentary condition, and justifies its
position among Aspidobranchia. At the same time it possesses a
totally distinct series of _functional_ gills, which are not derived
from the modification of the typical molluscan ctenidium. These gills
are in the form of delicate lamellae (fig. 4, f), which form a series
extending completely round the inner face of the depending
mantle-skirt. This circlet of gill-lamellae led Cuvier to class the
limpets as Cyclobranchiata, and, by erroneous identification of them
with the series of metamerically repeated ctenidia of _Chiton_, to
associate the latter mollusc with the former. The gill-lamellae of
_Patella_ are processes of the mantle comparable with the plait-like
folds often observed on the roof of the branchial chamber in other
Gastropoda (e.g. _Buccinum_ and _Haliotis_). They are termed pallial
gills. The only other molluscs in which they are exactly represented
are the curious Opisthobranchs _Phyllidia_ and _Pleurophyllidia_ (fig.
55). In these, as in _Patella_, the typical ctenidia are aborted, and
the branchial function is assumed by close-set lamelliform processes
arranged in a series beneath the mantle-skirt on either side of the
foot. In fig. 4, d, the large branchial vein of _Patella_ bringing
blood from the gill-series to the heart is seen; where it crosses the
series of lamellae there is a short interval devoid of lamellae.
[Illustration: Fig. 8.--A, Section in a plane vertical to the surface
of the neck of _Patella_ through a, the rudimentary ctenidium
(Lankester's organ), and b, the olfactory epithelium (osphradium); c,
the olfactory (osphradial) ganglion. (After Spengel.)
B, Surface view of a rudimentary ctenidium of _Patella_ excised and
viewed as a transparent object. (Lankester.)]
[Illustration: Fig. 9.--Nervous system of _Patella_; the visceral loop
is lightly shaded; the buccal ganglia are omitted. (After Spengel.)
ce, Cerebral ganglia.
c'e, Cerebral commissure.
pl, Pleural ganglion.
pe, Pedal ganglion. p'e, Pedal nerve.
s, s', Nerves (right and left) to the mantle.
o, Olfactory ganglion, connected by nerve to the streptoneurous
visceral loop.]
The heart in _Patella_ consists of a single auricle (not two as in
_Haliotis_ and _Fissurella_) and a ventricle; the former receives the
blood from the branchial vein, the latter distributes it through a
large aorta which soon leads into irregular blood-lacunae.
The existence of two renal organs in _Patella_, and their relation to
the pericardium (a portion of the coelom), is important. Each renal
organ is a sac lined with glandular epithelium (ciliated cell, with
concretions) communicating with the exterior by its papilla, and by a
narrow passage with the pericardium. The connexion with the
pericardium of the smaller of the two renal organs was demonstrated by
Lankester in 1867, at a time when the fact that the renal organ of the
Mollusca, as a rule, opens into the pericardium, and is therefore a
typical nephridium, was not known. Subsequent investigations carried
on under the direction of the same naturalist have shown that the
larger as well as the smaller renal sac is in communication with the
pericardium. The walls of the renal sacs are deeply plaited and thrown
into ridges. Below the surface these walls are excavated with
blood-vessels, so that the sac is practically a series of
blood-vessels covered with renal epithelium, and forming a meshwork
within a space communicating with the exterior. The larger renal sac
(remarkably enough, that which is aborted in other Anisopleura)
extends between the liver and the integument of the visceral dome very
widely. It also bends round the liver as shown in fig. 12, and forms
a large sac on half of the upper surface of the muscular mass of the
foot. Here it lies close upon the genital body (ovary or testis), and
in such intimate relationship with it that, when ripe, the gonad
bursts into the renal sac, and its products are carried to the
exterior by the papilla on the right side of the anus (Robin, Dall).
This fact led Cuvier erroneously to the belief that a duct existed
leading from the gonad to this papilla. The position of the gonad,
best seen in the diagrammatic section (fig. 13), is, as in other
Aspidobranchia, devoid of a special duct communicating with the
exterior. This condition, probably an archaic one, distinguishes the
Aspidobranchia from other Gastropoda.
[Illustration: FIG. 10.--Nervous system of _Haliotis_; the visceral
loop is lightly shaded; the buccal ganglia are omitted. (After
Spengel.)
ce, Cerebral ganglion.
pl.pe, The fused pleural and pedal ganglia.
pe, The right pedal nerve.
ce.pl, The cerebro-pleural connective.
ce.pe, The cerebro-pedal connective.
s, s', Right and left mantle nerves.
ab, Abdominal ganglion or site of same.
o, o, Right and left olfactory ganglia and osphardia receiving nerve
from visceral loop.]
[Illustration: FIG. 11.--Nervous system of _Fissurella_. (From
Gegenbaur, after Jhering.)
pl, Pallial nerve.
p, Pedal nerve.
A, Abdominal ganglia in the streptoneurous visceral commissure, with
supra- and sub-intestine ganglion on each side.
B, Buccal ganglia.
C, C, Cerebral ganglia.
es, Cerebral commissure.
o, Otocysts attached to the cerebro-pedal connectives.]
[Illustration: FIG. 12.--Diagram of the two renal organs (nephridia),
to show their relation to the rectum and to the pericardium.
(Lankester.)
f, Papilla of the larger nephridium.
g, Anal papilla with rectum leading from it.
h, Papilla of the smaller nephridium, which is only represented by
dotted outlines.
l, Pericardium indicated by a dotted outline--at its right side are
seen the two reno-pericardial pores.
ff, The sub-anal tract of the large nephridium given off near its
papilla and seen through the unshaded smaller nephridium.
ks.a, Anterior superior lobe of the large nephridium.
ks.l, Left lobe of same.
ks.p, Posterior lobe of same.
ks.i, Inferior sub-visceral lobe of same.]
[Illustration: FIG. 13.--Diagram of a vertical antero-postero median
section of a Limpet. Letters as in figs. 6, 7, with following
additions. (Lankester.)
q, Intestine in transverse section.
r, Lingual sac (radular sac).
rd, Radula.
s, Lamellated stomach.
t, Salivary gland.
u, Duct of same.
v, Buccal cavity
w, Gonad.
br.a, Branchial advehent vessel (artery).
br.v, Branchial efferent vessel (vein).
bv, Blood-vessel.
odm, Muscles and cartilage of the odontophore.
cor, Heart within the pericardium.]
[Illustration: FIG. 14.--Vertical section in a plane running right and
left through the anterior part of the visceral hump of _Patella_ to
show the two renal organs and their openings into the pericardium.
(J.T. Cunningham.)
a, Large or external or right renal organ.
ab, Narrow process of the same running _below_ the intestine and
leading by k into the pericardium.
b, Small or median renal organ.
c, Pericardium.
d, Rectum.
e, Liver.
f, Manyplies.
g, Epithelium of the dorsal surface.
h, Renal epithelium lining the renal sacs.
i, Aperture connecting the small sac with the pericardium.
k, Aperture connecting the large sac with the pericardium.]
The digestive tract of _Patella_ offers some interesting features. The
odontophore is powerfully developed; the radular sac is
extraordinarily long, lying coiled in a space between the mass of the
liver and the muscular foot. The radula has 160 rows of teeth with
twelve teeth in each row. Two pairs of salivary ducts, each leading
from a salivary gland, open into the buccal chamber. The oesophagus
leads into a remarkable stomach, plaited like the manyplies of a
sheep, and after this the intestine takes a very large number of turns
embedded in the yellow liver, until at last it passes between the two
renal sacs to the anal papilla. A curious ridge (spiral? valve) which
secretes a slimy cord is found upon the inner wall of the intestine.
The general structure of the Molluscan intestine has not been
sufficiently investigated to render any comparison of this structure
of _Patella_ with that of other Mollusca possible. The eyes of the
limpet deserve mention as examples of the most primitive kind of eye
in the Molluscan series. They are found one on each cephalic tentacle,
and are simply minute open pits or depressions of the epidermis, the
epidermic cells lining them being pigmented and connected with nerves
(compare fig. 14, art. CEPHALOPODA). The limpet breeds upon the
southern English coast in the early part of April, but its development
has not been followed. It has simply been traced as far as the
formation of a diblastula which acquires a ciliated band, and becomes
a nearly spherical trochosphere. It is probable that the limpet takes
several years to attain full growth, and during that period it
frequents the same spot, which becomes gradually sunk below the
surrounding surface, especially if the rock be carbonate of lime. At
low tide the limpet (being a strictly intertidal organism) is exposed
to the air, and (according to trustworthy observers) quits its
attachment and walks away in search of food (minute encrusting algae),
and then once more returns to the identical spot, not an inch in
diameter, which belongs, as it were, to it. Several million
limpets--twelve million in Berwickshire alone--are annually used on
the east coast of Britain as bait.
Sub-order 1. _Docoglossa._--Nervous system without dialyneury. Eyes
are open invaginations without crystalline lens. Two osphradia present
but no hypobranchial glands nor operculum. Teeth of radula beam-like,
and at most three marginal teeth on each side. Heart has only a single
auricle, neither heart nor pericardium traversed by rectum. Shell
conical without spire.
Fam. 1.--_Acmaeidae._ A single bipectinate ctenidium on left side.
Acmaea, without pallial branchiae, British. Scurria, with pallial
branchiae in a circle beneath the mantle.
Fam. 2.--_Tryblidiidae._ Muscle scar divided into numerous
impressions. _Tryblidium_, Silurian.
Fam. 3.--_Patellidae_. No ctenidia but pallial branchiae in a circle
between mantle and foot. _Patella_, pallial branchiae forming a
complete circle, no epipodial tentacles, British. _Ancistromesus_,
radula with median central tooth. _Nacella_, epipodial tentacles
present. _Helcion_, circlet of branchiae interrupted anteriorly,
British.
Fam. 4.--_Lepetidae._ Neither ctenidia nor pallial branchiae.
_Lepeta_, without eyes. _Pilidium. Propilidium._
Fam. 5.--_Bathysciadidae._ Hermaphrodite; head with appendage on
right side; radula without central tooth. _Bathysciadium_, abyssal.
Sub-order 2. RHIPIDOGLOSSA.--Aspidobranchia with a palliovisceral
anastomosis (dialyneurous); eye-vesicle closed, with crystalline lens;
ctenidia, osphradia and hypobranchial glands paired or single. Radula
with very numerous marginal teeth arranged like the rays of a fan.
Heart with two auricles; ventricle traversed by the rectum, except in
the _Helicinidae_. An epipodial ridge on each side of the foot and
cephalic expansions between the tentacles often present.
Fam. 1.--_Pleurotomariidae_. Shell spiral; mantle and shell with an
anterior fissure; two ctenidia; a horny operculum. _Pleurotomaria_,
epipodium without tentacles. Genus includes several hundred extinct
species ranging from the Silurian to the Tertiary. Five living
species from the Antilles, Japan and the Moluccas. Moluccan species
is 19 cm. in height.
Fam. 2.--_Bellerophontidae._ 300 species, all fossil, from Cambrian
to Trias.
Fam. 3.--_Euomphalidae._ Also extinct, from Cambrian to Cretaceous.
Fam. 4.--_Haliotidae._ Spire of shell much reduced; two bipectinate
ctenidia, the right being the smaller; no operculum. Haliotis.
Fam. 5.--_Velainiellidae_, an extinct family from the Eocene.
[Illustration: FIG. 15.--_Halio tistuberculata._ d, Foot; i,
tentacular processes of the mantle. (From Owen, after Cuvier.)]
Fam. 6.--_Fissurellidae._ Shell conical; slit or hole in anterior
part of mantle; two symmetrical ctenidia; no operculum.
_Emarginula_, mantle and shell with a slit, British. _Scutum_,
mantle split anteriorly and reflected over shell, which has no slit.
_Puncturella_, mantle and shell with a foramen in front of the apex,
British. _Fissurella_, mantle and shell perforated at apex, British.
Fam. 7.--_Cocculinidae._ Shell conical, symmetrical, without slit or
perforation. _Cocculina_, abyssal.
Fam. 8.--_Trochidae._ Shell spirally coiled; a single ctenidium;
eyes perforated; a horny operculum; lobes between the tentacles.
_Trochus_, shell umbilicated, spire pointed and prominent, British.
_Monodonta_, no jaws, spire not prominent, no umbilicus, columella
toothed. _Gibbula_, with jaws, three pairs of epipodial cirri
without pigment spots at their bases, British. _Margarita_, five to
seven pairs of epipodial cirri with a pigment spot at base of each.
[Illustration: FIG. 16.--_Scutum_, seen from the pedal surface.
(Lankester.)
o, Mouth.
T, Cephalic tentacle.
br, One of the two symmetrical gills placed on the neck.]
[Illustration: FIG. 17.--Dorsal aspect of a specimen of _Fissurella_
from which the shell has been removed, whilst the anterior area of the
mantle-skirt has been longitudinally slit and its sides reflected.
(Lankester.)
a, Cephalic tentacle.
b, Foot.
d, Left (archaic right) gill-plume.
e, Reflected mantle-flap.
fi, The fissure or hole in the mantle-flap traversed by the
longitudinal incision.
f, Right (archaic left) nephridium's aperture.
g, Anus.
h, Left (archaic right) aperture of nephridium.
p, Snout.]
Fam. 9.--_Stomatellidae._ Spire of shell much reduced; a single
ctenidium. _Stomatella_, foot truncated posteriorly, an operculum
present, no epipodial tentacles. _Gena_, foot elongated posteriorly,
no operculum.
Fam. 10.--_Delphinulidae._ Shell spirally coiled; operculum horny;
intertentacular lobes absent. _Delphinula._
Fam. 11.--_Liotiidae_, shell globular, margin of aperture thickened.
_Liotia_.
Fam. 12.--_Cyclostrematidae._ Shell flattened, umbilicated; foot
anteriorly truncated with angles produced into lobes. _Cyclostrema._
_Teinostoma._
Fam. 13.--_Trochonematidae._ All extinct, Cambrian to Cretaceous.
Fam. 14.--_Turbinidae._ Shell spirally coiled; epipodial tentacles
present; operculum thick and calcareous. _Turbo. Astralium. Molleria.
Cyclonema._
Fam. 15.--_Phasianellidae._ Shell not nacreous, without umbilicus,
with prominent spire and polished surface. _Phasianella._
Fam. 16.--_Umboniidae._ Shell flattened, not umbilicated, generally
smooth; operculum horny. _Umbonium. Isanda._
Fam. 17.--_Neritopsidae._ Shell semi-globular, with short spire;
operculum calcareous, not spiral. _Neritopsis. Naticopsis_,
extinct.
Fam. 18.--_Macluritidae._ Extinct, Cambrian and Silurian.
Fam. 19.--_Neritidae._ Shell with very low spire, without umbilicus,
internal partitions frequently absorbed; a single ctenidium; a
cephalic penis present. _Nerita_, marine. _Neritina_, freshwater,
British. _Septaria_, shell boat-shaped.
Fam. 20.--_Titiscaniidae._ Without shell and operculum, but with
pallial cavity and ctenidium. _Titiscania_, Pacific.
Fam. 21.--_Helicinidae._ No ctenidium, but a pulmonary cavity; heart
with a single auricle, not traversed by the rectum. _Helicina.
Eutrochatella. Stoastoma. Bourceria._
Fam. 22.--_Hydrocenidae._ No ctenidium, but a pulmonary cavity;
operculum with an apophysis. _Hydrocena_, Dalmatia.
Fam. 23.--_Proserpinidae._ No operculum. _Proserpina_, Central
America.
Order 2. PECTINIBRANCHIA.--In this order there is no longer any trace
of bilateral symmetry in the circulatory, respiratory and excretory
organs, the topographically right half of the pallial complex having
completely disappeared, except the right kidney, which is represented
by the genital duct. There is usually a penis in the male. The
ctenidium is monopectinate and attached to the mantle along its whole
length, except in _Adeorbis_ and _Valvata_; in the latter alone it is
bipectinate. There is a single well-developed, often pectinated
osphradium. The eye is always a closed vesicle, and the internal
cornea is extensive. In the radula there is a single central tooth or
none.
[Illustration: FIG. 18.--Animal and shell of _Pyrula laevigata_. (From
Owen.)
a, Siphon.
b, Head-tentacles.
C, Head, the letter placed near the right eye.
d, The foot, expanded as in crawling.
h, The mantle-skirt reflected over the sides of the shell.]
The former classification into Holochlamyda, Pneumochlamyda and
Siphonochlamyda has been abandoned, as it was founded on adaptive
characters not always indicative of true affinities. The order is now
divided into two sub-orders: the Taenioglossa, in which there are
three teeth on each side of the median tooth of the radula, and the
Stenoglossa, in which there is only one tooth on each side of the
median tooth. In the latter a pallial siphon, a well-developed
proboscis and an unpaired oesophageal gland are always present, in the
former they are usually absent. The siphon is an incompletely tubular
outgrowth of the mantle margin on the left side, contained in a
corresponding outgrowth of the edge of the shell-mouth, and serving to
conduct water to the respiratory cavity.
The condition usually spoken of as a "proboscis" appears to be derived
from the condition of a simple rostrum (having the mouth at its
extremity) by the process of _incomplete introversion_ of that simple
rostrum. There is no reason in the actual significance of the word why
the term "proboscis" should be applied to an alternately introversible
and eversible tube connected with an animal's body, and yet such is a
very customary use of the term. The introversible tube may be
completely closed, as in the "proboscis" of Nemertine worms, or it may
have a passage in it leading into a non-eversible oesophagus, as in
the present case, and in the case of the eversible pharynx of the
predatory Chaetopod worms. The diagrams here introduced (fig. 19) are
intended to show certain important distinctions which obtain amongst
the various "introverts," or intro- and e-versible tubes so frequently
met with in animal bodies. Supposing the tube to be completely
introverted and to commence its eversion, we then find that eversion
may take place, either by a forward movement of the side of the tube
near its attached base, as in the proboscis of the Nemertine worms,
the pharynx of Chaetopods and the eye-tentacle of Gastropods, or by a
forward movement of the inverted apex of the tube, as in the proboscis
of the Rhabdocoel Planarians, and in that of Gastropods here under
consideration. The former case we call "pleurecbolic" (fig. 19, A, B,
C, H, I, K), the latter "acrecbolic" tubes or introverts (fig. 19, D,
E, F, G). It is clear that, if we start from the condition of full
eversion of the tube and watch the process of introversion, we shall
find that the pleurecbolic variety is introverted by the apex of the
tube sinking inwards; it may be called acrembolic, whilst conversely
the acrecbolic tubes are pleurembolic. Further, it is obvious enough
that the process either of introversion or of eversion of the tube may
be arrested at any point, by the development of fibres connecting the
wall of the introverted tube with the wall of the body, or with an
axial structure such as the oesophagus; on the other hand, the range
of movement of the tubular introvert may be unlimited or complete. The
acrembolic proboscis or frontal introvert of the Nemertine worms has a
complete range. So has the acrembolic pharynx of Chaetopods, if we
consider the organ as terminating at that point where the jaws are
placed and the oesophagus commences. So too the acrembolic
eye-tentacle of the snail has a complete range of movement, and also
the pleurembolic proboscis of the Rhabdocoel prostoma. The introverted
rostrum of the Pectinibranch Gastropods presents in contrast to these
a limited range of movement. The "introvert" in these Gastropods is
not the pharynx as in the Chaetopod worms, but a prae-oral structure,
its apical limit being formed by the true lips and jaws, whilst the
apical limit of the Chaetopod's introvert is formed by the jaws placed
at the junction of pharynx and oesophagus, so that the Chaetopod's
introvert is part of the stomodaeum or fore-gut, whilst that of the
Gastropod is external to the alimentary canal altogether, being in
front of the mouth, not behind it, as is the Chaetopod's. Further, the
Gastropod's introvert is pleurembolic (and therefore acrecbolic), and
is limited both in eversion and in introversion; it cannot be
completely everted owing to the muscular bands (fig. 19, G), nor can
it be fully introverted owing to the bands (fig. 19, F) which tie the
axial pharynx to the adjacent wall of the apical part of the
introvert. As in all such intro- and e-versible organs, eversion of
the Gastropod proboscis is effected by pressure communicated by the
muscular body-wall to the liquid contents (blood) of the body-space,
accompanied by the relaxation of the muscles which directly pull upon
either the sides or the apex of the tubular organ. The inversion of
the proboscis is effected directly by the contraction of these
muscles. In various members of the Pectinibranchia the mouth-bearing
cylinder is introversible (i.e. is a _proboscis_)--with rare
exceptions these forms have a siphonate mantle-skirt. On the other
hand, many which have a siphonate mantle-skirt are not provided with
an introversible mouth-bearing cylinder, but have a simple
non-introversible rostrum, as it has been termed, which is also the
condition presented by the mouth-bearing region in nearly all other
Gastropoda. One of the best examples of the introversible
mouth-cylinder or proboscis which can be found is that of the common
whelk (_Buccinum undatum_) and its immediate allies. In fig. 23 the
proboscis is seen in an everted state; it is only so carried when
feeding, being withdrawn when the animal is at rest. Probably its use
is to enable the animal to introduce its rasping and licking apparatus
into very narrow apertures for the purposes of feeding, e.g. into a
small hole bored in the shell of another mollusc.
[Illustration: FIG. 19.--Diagrams explanatory of the nature of
so-called proboscides or "introverts." (Lankester.)
A, Simple introvert completely introverted.
B, The same, partially everted by eversion of the sides, as in the
Nemertine proboscis and Gastropod eye-tentacle = pleurecbolic.
C, The same, fully everted.
D, E, A similar simple introvert in course of eversion by the
forward movement, not of its sides, but of its apex, as in the
proboscidean Rhabdocoels = acrecbolic.
F, Acrecbolic (= pleurembolic) introvert, formed by the snout of the
proboscidiferous Gastropod. al, alimentary canal; d, the true mouth.
The introvert is not a simple one with complete range both in
eversion and introversion, but is arrested in introversion by the
fibrous bands at c, and similarly in eversion by the fibrous bands
at b.
G, The acrecbolic snout of a proboscidiferous Gastropod, arrested
short of complete eversion by the fibrous band b.
H, The acrembolic (= pleurecbolic) pharynx of a Chaetopod fully
introverted. al, alimentary canal; at d, the jaws; at a, the mouth;
therefore a to d is stomodaeum, whereas in the Gastropod (F) a to d
is inverted body-surface.
I, Partial eversion of H.