Chapter II: Part 2

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.]

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.

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.

o, Mouth.
T, Cephalic tentacle.
br, One of the two symmetrical gills placed on the neck.]

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.

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.

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.

K, Complete eversion of H.]

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."]

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.

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.

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).

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.]

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.

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.

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).

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.

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._

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.

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_.

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._

Fam. 51.--_Solariidae. Solarium. Torinia. Fluxina._

Fam. 52.--_Scalariidae._ Shell turriculated, with elongated spire;
proboscis short; siphon rudimentary. _Scalaria. Eglisia._ Crossea.
Aclis.

The three following families have neither radula nor jaws, and are
therefore called _Aglossa_. They have a well-developed proboscis which
is used as a suctorial organ; some are abyssal, but the majority are
either commensals or parasites of Echinoderms.

Fam. 53.--_Pyramidellidae._ Summit of spire heterostrophic; a
projection, the mentum, between head and foot; operculum present.
_Pyramidella. Turbonilla. Odostomia_, British. _Myxa._

a, Apex.
ac, Siphonal notch of the mouth of the shell.
ac to pc, Mouth of the shell.
w, w, Whorls of the shell.
s, s. Sutures.

Occupying the axis, and exposed by the section, is seen the
"columella" or spiral pillar. The upper whorls of the shell are
seen to be divided into separate chambers by the formation of
successively formed "septa."]

Fam. 54.--_Eulimidae._ Visceral mass still coiled spirally; shell
thin and shining. _Eulima_, foot well developed, with an operculum,
animal usually free, but some live in the digestive cavity of
Holothurians. _Mucronalia_, foot reduced, but still operculate, eyes
present, animal fixed by its very long proboscis which is deeply
buried in the tissues of an Echinoderm, no pseudopallium.
_Stylifer_, the operculum is lost, animal fixed by a large proboscis
which forms a pseudopallium covering the whole shell except the
extremity of the spire, parasitic on all groups of Echinoderms.
_Entosiphon_, visceral mass still coiled; shell much reduced,
proboscis very long forming a pseudopallium which covers the whole
body and projects beyond in the form of a siphon, foot and nervous
system present, eyes, branchia and anus absent, parasite in the
Holothurian _Deima blakei_ in the Indian Ocean.

Fam. 55.--_Entoconchidae._ No shell; visceral mass not coiled; no
sensory organs, nervous system, branchia or anus; body reduced to a
more or less tubular sac; hermaphrodite and viviparous; parasitic in
Holothurians; larvae are veligers, with shell and operculum.
_Entocolax_, mouth at free extremity, animal fixed by aboral orifice
of pseudopallium, Pacific. _Entoconcha_, body elongated and tubular,
animal fixed by the oral extremity, protandric hermaphrodite,
parasitic in testes of Holothurians causing their abortion.
_Enteroxenos_, no pseudopallium and no intestine, hermaphrodite,
larvae with operculum.

Tribe 2.--HETEROPODA. Pelagic Taenioglossa with foot large and
laterally compressed to form a fin.

Fam. 1. _Atlantidae._ Visceral sac and shell coiled in one plane;
foot divided transversely into two parts, posterior part bearing an
operculum, anterior part forming a fin provided with a sucker.
_Atlanta. Oxygyrus._

Fam. 2.--_Carinariidae._ Visceral sac and shell small in proportion
to the rest of the body, which cannot be withdrawn into the shell;
foot elongated, fin-shaped, with sucker, but without operculum.
_Carinaria. Cardiopoda._

Fam. 3.--_Pterotrachaeidae._ Visceral sac very much reduced; without
shell or mantle; anus posterior; foot provided with sucker in male
only. _Pterotrachaea. Firoloida. Pterosoma._

Sub-order 2.--STENOGLOSSA. Radula narrow with one lateral tooth on
each side, and one median tooth or none.

Tribe 1.--RACHIGLOSSA. Radula with a median tooth and a single tooth
on each side of it. Formula 1 : 1 : 1. Rudimentary jaws present.

Fam. 1.--_Turbinellidae._ Shell solid, piriform, with thick folded
columella; lateral teeth of radula bicuspidate. _Turbinella.
Cynodonta. Fulgur. Hemifusus. Tudicla. Strepsidura._

Fam. 2.--_Fasciolariidae._ Shell elongated, with long siphon;
lateral teeth of radula multicuspidate. _Fasciolaria. Fusus.
Clavella. Latirus._

Fam. 3.--_Mitridae._ Shell fusiform and solid, aperture elongated,
columella folded; no operculum; eyes on sides of tentacles. _Mitra.
Turricula. Cylindromitra. Imbricaria._

Fam. 4.--_Buccinidae._ Foot large and broad; eyes at base of
tentacles; operculum horny. _Buccinum. Chrysodomus. Liomesus.
Cominella. Tritonidea. Pisania. Euthria. Phos. Dipsacus._

Fam. 5.--_Nassidae._ Foot broad, with two slender posterior
appendages; operculum unguiculate. _Nassa_, marine, British.
_Canidia_, fluviatile. _Bullia._

Fam. 6.--_Muricidae._ Shell with moderately long spire and canal,
ornamented with ribs, often spiny; foot truncated anteriorly.
_Murex_, British. _Trophon_, British. _Typhis. Urosalpinx.
Lachesis._

Fam. 7.--_Purpuridae._ Shell thick, with short spire, last whorl
large and canal short; aperture wide; operculum horny. _Purpura_,
British. _Rapana. Monoceros. Sistrum. Concholepas._

Fam. 8.--_Haliidae._ Shell ventricose, thin and smooth, with wide
aperture; foot large and thick, without operculum. _Halia._

Fam. 9.--_Cancellariidae._ Shell ovoid, with short spire and folded
columella; foot small, no operculum; siphon short. _Cancellaria._

Fam. 10.--_Columbellidae._ Spire of shell prominent, aperture
narrow, canal very short, columella crenelated; foot large.
_Columbella._

Fam. 11.--_Coralliophilidae._ Shell irregular; radula absent; foot
and siphon short; sedentary animals, living in corals.
_Coralliophila. Rhizochilus. Leptoconchus. Magilus. Rapa._

Fam. 12.--_Volutidae._ Head much flattened and wide, with eyes on
sides; foot broad; siphon with internal appendages. _Valuta.
Guivillea. Cymba._

Fam. 13.--_Olividae._ Foot with anterior transverse groove; a
posterior pallial tentacle; generally burrowing. _Olivia. Olivella.
Ancillaria. Agaronia._

Fam. 14.--_Marginellidae._ Foot very large; mantle reflected over
shell. _Marginella. Pseudomarginella._

Fam. 15.--_Harpidae._ Foot very large; without operculum; shell with
short spire and longitudinal ribs; siphon long. _Harpa._

Tribe 2.--TOXIGLOSSA. No jaws. No median tooth in radula. Formula: 1 :
0 : 1. Poison-gland present whose duct traverses the nerve-collar.

Fam. 1.--_Pleurotomatidae._ Shell fusiform, with elongated spire;
margin of shell and mantle notched. _Pleurotoma. Clavatula.
Mangilia. Bela. Pusionella. Pontiothauma._

Fam. 2.--_Terebridae._ Shell turriculated, with numerous whorls;
aperture and operculum oval; eyes at summits of tentacles; siphon
long. _Terebra._

Fam. 3.--_Conidae._ Shell conical, with very short spire, and narrow
aperture with parallel borders; operculum unguiform _Conus._

Sub-Class II.--EUTHYNEURA

The most important general character of the Euthyneura is the absence of torsion in the visceral commissure, and the more posterior position of the anus and pallial organs. Comparative anatomy and embryology prove that this condition is due, not as formerly supposed to a difference in the relations of the visceral commissure which prevented it from being included in the torsion of the visceral hump, but to an actual detorsion which has taken place in evolution and is repeated to a great extent in individual development. In several of the more primitive forms the same torsion occurs as in Streptoneura, viz. in _Actaeon_ and _Limacina_ among Opisthobranchia, and _Chilina_ among Pulmonata. _Actaeon_ is proso-branchiate, the visceral commissure is twisted in _Actaeon_ and _Chilina_, and even slightly still in _Bulla_ and _Scaphander_; in _Actaeon_ and _Limacina_ the osphradium is to the left, innervated by the supra-intestinal ganglion. But in the other members of the sub-class the detorsion of the visceral mass has carried back the anus and circumanal complex from the anterior dorsal region to the right side, as in _Bulla_ and _Aplysia_, or even to the posterior end of the body, as in _Philine, Oncidium, Doris_, &c. Different degrees of the same process of detorsion are, as we have seen, exhibited by the Heteropoda among the Streptoneura, and both in them and in the Euthyneura the detorsion is associated with degeneration of the shell. Where the modification is carried to its extreme degree, not only the shell but the pallial cavity, ctenidium and visceral hump disappear, and the body acquires a simple elongated form and a secondary external symmetry, as in _Pterotrachaea_ and in _Doris, Eolis_, and other Nudibranchia. These facts afford strong support to the hypothesis that the weight of the shell is the original cause of the torsion of the dorsal visceral mass in Gastropods. But this hypothesis leaves the elevation of the visceral mass and the exogastric coiling of the shell in the ancestral form unexplained. In those Euthyneura in which the shell is entirely absent in the adult, it is, except in the three genera _Cenia, Runcina_ and _Vaginula_, developed in the larva and then falls off. In other cases (Tectibranchs) the reduced shell is enclosed by upgrowths of the edge of the mantle and becomes internal, as in many Cephalopods. A few Euthyneura in which the shell is not much reduced retain an operculum in the adult state, e.g. _Actaeon, Limacina_, and the marine Pulmonate, _Amphibola_. The detorted visceral commissure shows a tendency to the concentration of all its elements round the oesophagus, so that except in the Bullomorpha and in _Aplysia_ the whole nervous system is aggregated in the cephalic region, either dorsally or ventrally. The radula has a number of uniform teeth on each side of the median tooth in each transverse row. The head in most cases bears two pairs of tentacles. All the Euthyneura are hermaphrodite.

In the most primitive condition the genital duct is single throughout its length and has a single external aperture; it is therefore said to be monaulic. The hermaphrodite aperture is on the right side near the opening of the pallial cavity, and a ciliated groove conducts the spermatozoa to the penis, which is situated more anteriorly. This is the condition in the Bullomorpha, the Aplysiomorpha, and in one Pulmonate, _Pythia_. In some cases while the original aperture remains undivided, the seminal groove is closed and so converted into a canal. This is the modification found in _Cavolinia longirostris_ among the Bullomorpha, and in all the _Auriculidae_ except _Pythia_. A further degree of modification occurs when the male duct takes its origin from the hermaphrodite duct above the external opening, so that there are two distinct apertures, one male and one female, the latter being the original opening. The genital duct is now said to be diaulic, as in _Valvata, Oncidiopsis, Actaeon_, and _Lobiger_ among the Bullomorpha, in the _Pleurobranchidae_, in the Nudibranchia, except the Doridomorpha and most of the Elysiomorpha, and in the Pulmonata. Originally in this condition the female aperture is at some distance from the male, as in the Basommatophora and in other cases; but in some forms the female aperture itself has shifted and come to be contiguous with the male opening and penis as in the Stylommatophora. In all these cases the female duct bears a bursa copulatrix or receptaculum seminis. In some forms this receptacle acquires a separate external opening remaining connected with the oviduct internally. There are thus two female openings, one for copulation, the other for oviposition, as well as a male opening. The genital duct is now trifurcated or triaulic, a condition which is confined to certain Nudibranchs, viz. the Doridomorpha and most of the Elysiomorpha.

The Pteropoda, formerly regarded as a distinct class of the Mollusca, were interpreted by E.R. Lankester as a branch of the Cephalopoda, chiefly on account of the protrusible sucker-bearing processes at the anterior end of _Pneumonoderma_. These he considered to be homologous with the arms of Cephalopods. He fully recognized, however, the similarity of Pteropods to Gastropods in their general asymmetry and in the torsion of the visceral mass in _Limacinidae_. It is now understood that they are Euthyneurous Gastropods adapted to natatory locomotion and pelagic life. The sucker-bearing processes of _Pneumonoderma_ are outgrowths of the proboscis. The fins of Pteropods are now interpreted as the expanded lateral margins of the foot, termed parapodia, not homologous with the siphon of Cephalopods which is formed from epipodia. The Thecosomatous Pteropoda are allied to _Bulla_, the Gymnosomatous forms to _Aplysia_. The Euthyneura comprises two orders, Opisthobranchia and Pulmonata.

A, Veliger-larva of an Opisthobranch (_Polycera_). f, Foot; op,
operculum; mn, anal papilla; ry, dry, two portions of unabsorbed
nutritive yolk on either side of the intestine. The right otocyst is
seen at the root of the foot.

B, Trochosphere of an Opisthobranch (_Pleurobranchidium_)
showing--shgr, the shell-gland or primitive shell-sac; v, the cilia
of the velum; ph, the commencing stomodaeum or oral invagination;
ot, the left otocyst; pg, red-coloured pigment spot.

C, Diblastula of an Opisthobranch (_Polycera_) with elongated
blastopore oi.

(All from Lankester.)]