Chapter III: Part 3

Order 1.--OPISTHOBRANCHIA. Marine Euthyneura, the more archaic forms
of which have a relatively large foot and a small visceral hump, from
the base of which projects on the right side a short mantle-skirt. The
anus is placed in such forms far back beyond the mantle-skirt. In
front of the anus, and only partially covered by the mantle-skirt, is
the ctenidium with its free end turned backwards. The heart lies in
front of, instead of to the side of, the attachment of the
ctenidium--hence Opisthobranchia as opposed to "Prosobranchia," which
correspond to the Streptoneura. A shell is possessed in the adult
state by but few Opisthobranchia, but all pass through a veliger
larval stage with a nautiloid shell (fig. 36). Many Opisthobranchia
have by a process of atrophy lost the typical ctenidium and the
mantle-skirt, and have developed other organs in their place. As in
some Pectinibranchia, the free margin of the mantle-skirt is
frequently reflected over the shell when a shell exists; and, as in
some Pectinibranchia, broad lateral outgrowths of the foot (parapodia)
are often developed which may be thrown over the shell or naked dorsal
surface of the body.

a, Mouth.
b, Radular sac.
c, Oesophagus.
d, Stomach.
c', Intestine.
f', Anus.
g, g', g", g"', The four lobes of the liver.
h, The heart (auricle and ventricle).
l, The renal sac (nephridium).
l', The ciliated communication of the renal sac with the pericardium.
m, The external opening of the renal sac.
n, The cerebral ganglion.
o, The cephalic tentacles.
f, The genital pore.
y, The ovo-testes.
w, The parasitic hydromedusa Mnestra, usually found attached in this
position by the aboral pole of its umbrella.]

The variety of special developments of structure accompanying the
atrophy of typical organs in the Opisthobranchia and general
degeneration of organization is very great. The members of the order
present the same wide range of superficial appearance as do the
Pectinibranchiate Streptoneura, forms carrying well-developed spiral
shells and large mantle-skirts being included in the group, together
with flattened or cylindrical slug-like forms. But in respect of the
substitution of other parts for the mantle-skirt and for the gill
which the more degenerate Opisthobranchia exhibit, this order stands
alone. Some Opisthobranchia are striking examples of degeneration
(some Nudibranchia), having none of those regions or processes of the
body developed which distinguish the archaic Mollusca from such
flat-worms as the Dendrocoel Planarians. Indeed, were it not for
their retention of the characteristic odontophore we should have
little or no indication that such forms as _Phyllirhoë_ and
_Limapontia_ really belong to the Mollusca at all. The interesting
little _Rhodope veranyii_, which has no odontophore, has been
associated by systematists both with these simplified Opisthobranchs
and with Rhabdocoel Planarians.

t, Anterior cephalic tentacles.
t², Posterior cephalic tentacles.
e, Eyes.
f, Metapodium.
ep, Epipodium.
g, Gill-plume (ctenidium).
m, Mantle-flap reflected over the thin oval shell.
os, s, Orifice formed by the unclosed border of the reflected
mantle-skirt, allowing the shell to show.
pe, The spermatic groove.]

In many respects the sea-hare (_Aplysia_), of which several species
are known (some occurring on the English coast), serves as a
convenient example of the fullest development of the organization
characteristic of Opisthobranchia. The woodcut (fig. 38) gives a
faithful representation of the great mobility of the various parts of
the body. The head is well marked and joined to the body by a somewhat
constricted neck. It carries two pairs of cephalic tentacles and a
pair of sessile eyes. The visceral hump is low and not drawn out into
a spire. The foot is long, carrying the oblong visceral mass upon it,
and projecting (as metapodium) a little beyond it (f). Laterally the
foot gives rise to a pair of mobile fleshy lobes, the parapodia (ep),
which can be thrown up so as to cover in the dorsal surface of the
animal. Such parapodia are common, though by no means universal, among
Opisthobranchia. The torsion of the visceral hump is not carried out
very fully, the consequence being that the anus has a posterior
position a little to the right of the median line above the
metapodium, whilst the branchial chamber formed by the overhanging
mantle-skirt faces the right side of the body instead of lying well to
the front as in Streptoneura and as in Pulmonate Euthyneura. The
gill-plume, which in _Aplysia_ is the typical Molluscan ctenidium, is
seen in fig. 39 projecting from the branchial sub-pallial space. The
relation of the delicate shell to the mantle is peculiar, since it
occupies an oval area upon the visceral hump, the extent of which is
indicated in fig. 38, C, but may be better understood by a glance at
the figures of the allied genus _Umbrella_ (fig. 40), in which the
margin of the mantle-skirt coincides, just as it does in the limpet,
with the margin of the shell. But in _Aplysia_ the mantle is reflected
over the edge of the shell, and grows over its upper surface so as to
completely enclose it, excepting at the small central area s where the
naked shell is exposed. This enclosure of the shell is a permanent
development of the arrangement seen in many Streptoneura (e.g.
_Pyrula, Ovula_, see figs. 18 and 32), where the border of the mantle
can be, and usually is, drawn over the shell, though it is withdrawn
(as it cannot be in _Aplysia_) when they are irritated. From the fact
that _Aplysia_ commences its life as a free-swimming veliger with a
nautiloid shell not enclosed in any way by the border of the mantle,
it is clear that the enclosure of the shell in the adult is a
secondary process. Accordingly, the shell of _Aplysia_ must not be
confounded with a primitive shell in its shell-sac, such as we find
realized in the shells of _Chiton_ and in the plugs which form in the
remarkable transitory "shell-sac" or "shell-gland" of Molluscan
embryos (see figs. 26, 60). _Aplysia_, like other Mollusca, develops a
primitive shell-sac in its trochosphere stage of development, which
disappears and is succeeded by a nautiloid shell (fig. 36). This forms
the nucleus of the adult shell, and, as the animal grows, becomes
enclosed by a reflection of the mantle-skirt. When the shell of an
_Aplysia_ enclosed in its mantle is pushed well to the left, the
sub-pallial space is fully exposed as in fig. 39, and the various
apertures of the body are seen. Posteriorly we have the anus, in
front of this the lobate gill-plume, between the two (hence
corresponding in position to that of the Pectinibranchia) we have the
aperture of the renal organ. In front, near the anterior attachment of
the gill-plume, is the osphradium (olfactory organ) discovered by J.W.
Spengel, yellowish in colour, in the typical position, and overlying
an olfactory ganglion with typical nerve-connexion (see fig. 43). To
the right of Spengel's osphradium is the opening of a peculiar gland
which has, when dissected out, the form of a bunch of grapes; its
secretion is said to be poisonous. On the under side of the free edge
of the mantle are situated the numerous small cutaneous glands which,
in the large _Aplysia camelus_ (not in other species), form the purple
secretion which was known to the ancients. In front of the osphradium
is the single genital pore, the aperture of the common or
hermaphrodite duct. From this point there passes forward to the right
side of the head a groove--the spermatic groove--down which the
spermatic fluid passes. In other Euthyneura this groove may close up
and form a canal. At its termination by the side of the head is the
muscular introverted penis. In the hinder part of the foot (not shown
in any of the diagrams) is the opening of a large mucus-forming gland
very often found in the Molluscan foot.

a, Anterior cephalic tentacle.
b, Posterior cephalic tentacle; between a and b, the eyes.
c, Right epipodium.
d, Left epipodium.
e, Hinder part of visceral hump.
fp, Posterior extremity of the foot.
fa, Anterior part of the foot underlying the head.
g, The ctenidium (branchial plume).
h, The mantle-skirt tightly spread over the horny shell and pushed
with it towards the left side.
i, The spermatic groove.
k, The common genital pore (male and female).
l, Orifice of the grape-shaped (supposed poisonous) gland.
m, The osphradium (olfactory organ of Spengel).
n, Outline of part of the renal sac (nephridium) below the surface.
o, External aperture of the nephridium.
p, Anus.]

With regard to internal organization we may commence with the
disposition of the renal organ (nephridium), the external opening of
which has already been noted. The position of this opening and other
features of the renal organ were determined by J.T. Cunningham.

There is considerable uncertainty with respect to the names of the
species of _Aplysia_. There are two forms which are very common in the
Gulf of Naples. One is quite black in colour, and measures when
outstretched 8 or 9 in. in length. The other is light brown and
somewhat smaller, its length usually not exceeding 7 in. The first is
flaccid and sluggish in its movements, and has not much power of
contraction; its epipodial lobes are enormously developed and extend
far forward along the body; it gives out when handled an abundance of
purple liquid, which is derived from cutaneous glands situated on the
under side of the free edge of the mantle. According to F. Blochmann
it is identical with _A. camelus_ of Cuvier. The other species is _A.
depilans_; it is firm to the touch, and contracts forcibly when
irritated; the secretion of the mantle-glands is not abundant, and is
milky white in appearance. The kidney has similar relations in both
species, and is identical with the organ spoken of by many authors as
the triangular gland. Its superficial extent is seen when the folds
covering the shell are cut away and the shell removed; the external
surface forms a triangle with its base bordering the pericardium, and
its apex directed posteriorly and reaching the the left-hand posterior
corner of the shell-chamber. The dorsal surface of the kidney extends
to the left beyond the shell-chamber beneath the skin in the space
between the shell-chamber and the left parapodium.

When the animal is turned on its left-hand side and the mantle-chamber
widely opened, the gill being turned over to the left, a part of the
kidney is seen beneath the skin between the attachment of the gill and
the right parapodium (fig. 39). On examination this is found to be the
under surface of the posterior limb of the gland, the upper surface of
which has just been described as lying beneath the shell. In the
posterior third of this portion, close to that edge which is adjacent
to the base of the gill, is the external opening (fig. 39, o).

When the pericardium is cut open from above in an animal otherwise
entire, the anterior face of the kidney is seen forming the posterior
wall of the pericardial chamber; on the deep edge of this face, a
little to the left of the attachment of the auricle to the floor of
the pericardium, is seen a depression; this depression contains the
opening from the pericardium into the kidney.

To complete the account of the relations of the organ: the right
anterior corner can be seen superficially in the wall of the
mantle-chamber above the gill. Thus the base of the gill passes in a
slanting direction across the right-hand side of the kidney, the
posterior end being dorsal to the apex of the gland, and the anterior
end ventral to the right-hand corner.

As so great a part of the whole surface of the kidney lies adjacent to
external surfaces of the body, the remaining part which faces the
internal organs is small; it consists of the left part of the under
surface; it is level with the floor of the pericardium, and lies over
the globular mass formed by the liver and convoluted intestine.

i, Ovo-testis.
h, Hermaphrodite duct.
g, Albuminiparous gland.
f, Vesicula seminalis.
k, Opening of the albuminiparous gland into the hermaphrodite duct.
e, Hermaphrodite duct (uterine portion).
b, Vaginal portion of the uterine duct.
c, Spermatheca.
d, Its duct.
a, Genital pore.]

Thus the renal organ of _Aplysia_ is shown to conform to the Molluscan
type. The heart lying within the adjacent pericardium has the usual
form, a single auricle and ventricle. The vascular system is not
extensive, the arteries soon ending in the well-marked spongy tissue
which builds up the muscular foot, parapodia, and dorsal body-wall.

The alimentary canal commences with the usual buccal mass; the lips
are cartilaginous, but not armed with horny jaws, though these are
common in other Opisthobranchs; the lingual ribbon is
multidenticulate, and a pair of salivary glands pour in their
secretion. The oesophagus expands into a curious gizzard, which is
armed internally with large horny processes, some broad and thick,
others spinous, fitted to act as crushing instruments. From this we
pass to a stomach and a coil of intestine embedded in the lobes of a
voluminous liver; a caecum of large size is given off near the
commencement of the intestine. The liver opens by two ducts into the
digestive tract.

The generative organs lie close to the coil of intestine and liver, a
little to the left side. When dissected out they appear as represented
in fig. 41. The essential reproductive organ or gonad consists of both
ovarian and testicular cells (see fig. 42). It is an ovo-testis. From
it passes a common or hermaphrodite duct, which very soon becomes
entwined in the spire of a gland--the albuminiparous gland. The latter
opens into the common duct at the point k, and here also is a small
diverticulum of the duct f. Passing on, we find not far from the
genital pore a glandular spherical body (the spermatheca c) opening by
means of a longish duct into the common duct, and then we reach the
pore (fig. 39, k). Here the female apparatus terminates. But when the
male secretion of the ovo-testis is active, the seminal fluid passes
from the genital pore along the spermatic groove (fig. 39) to the
penis, and is by the aid of that eversible muscular organ introduced
into the genital pore of a second _Aplysia_, whence it passes into the
spermatheca, there to await the activity of the female element of the
ovo-testis of this second _Aplysia_. After an interval of some
days--possibly weeks--the ova of the second _Aplysia_ commence to
descend the hermaphrodite duct; they become enclosed in a viscid
secretion at the point where the albuminiparous gland opens into the
duct intertwined with it; and on reaching the point where the
spermathecal duct debouches they are impregnated by the spermatozoa
which escape now from the spermatheca and meet the ova.

The development of _Aplysia_ from the egg presents many points of
interest from the point of view of comparative embryology, but in
relation to the morphology of the Opisthobranchia it is sufficient to
point to the occurrence of a trochosphere and a veliger stage (fig.
36), and of a shell-gland or primitive shell-sac (fig. 36, _shgr_),
which is succeeded by a nautiloid shell.

ce, Cerebral ganglion.
pl, Pleural ganglion.
pe, Pedal ganglion.
ab.sp, Abdominal ganglion which represents also the
supra-intestinal ganglion of Streptoneura and gives off the nerve
to the osphradium (olfactory organ) o, and another to an unlettered
so-called "genital" ganglion. The buccal nerves and ganglia are
omitted.]

In the nervous system of _Aplysia_ the great ganglion-pairs are well
developed and distinct. The euthyneurous visceral loop is long, and
presents only one ganglion (in _Aplysia camelus_, but two distinct
ganglia joined to one another in _Aplysia hybrida_ of the English
coast), placed at its extreme limit, representing both the right and
left visceral ganglia and the third or abdominal ganglion, which are
so often separately present. The diagram (fig. 43) shows the nerve
connecting this abdomino-visceral ganglion with the olfactory ganglion
of Spengel. It is also seen to be connected with a more remote
ganglion--the genital. Such special irregularities in the development
of ganglia upon the visceral loop, and on one or more of the main
nerves connected with it, are very frequent. Our figure of the nervous
system of _Aplysia_ does not give the small pair of buccal ganglia
which are, as in all glossophorous Molluscs, present upon the nerves
passing from the cerebral region to the odontophore.

For a comparison of various Opisthobranchs, _Aplysia_ will be found to
present a convenient starting-point. It is one of the more typical
Opisthobranchs, that is to say, it belongs to the section
Tectibranchia, but other members of the suborder, namely, _Bulla_ and
_Actaeon_ (figs. 44 and 45), are less abnormal than _Aplysia_ in
regard to their shells and the form of the visceral hump. They have
naked spirally twisted shells which may be concealed from view in the
living animal by the expansion and reflection of the parapodia, but
are not enclosed by the mantle, whilst _Actaeon_ is remarkable for
possessing an operculum like that of so many Streptoneura.

The great development of the parapodia seen in _Aplysia_ is usual in
Tectibranchiate Opisthobranchs. The whole surface of the body becomes
greatly modified in those Nudibranchiate forms which have lost, not
only the shell, but also the ctenidium. Many of these have peculiar
processes developed on the dorsal surface (fig. 46, A, B), or retain
purely negative characters (fig. 46, D). The chief modification of
internal organization presented by these forms, as compared with
_Aplysia_, is found in the condition of the alimentary canal. The
liver is no longer a compact organ opening by a pair of ducts into the
median digestive tract, but we find very numerous hepatic diverticula
on a shortened axial tract (fig. 47). These diverticula extend usually
one into each of the dorsal papillae or "cerata" when these are
present. They are not merely digestive glands, but are sufficiently
wide to act as receptacles of food, and in them the digestion of food
proceeds just as in the axial portion of the canal. A precisely
similar modification of the liver or great digestive gland is found in
the scorpions, where the axial portion of the digestive canal is short
and straight, and the lateral ducts sufficiently wide to admit food
into the ramifications of the gland there to be digested; whilst in
the spiders the gland is reduced to a series of simple caeca.

The typical character is retained by the heart, pericardium, and the
communicating nephridium or renal organ in all Opisthobranchs. An
interesting example of this is furnished by the fish-like transparent
_Phyllirhoë_ (fig. 37), in which it is possible most satisfactorily to
study in the living animal, by means of the microscope, the course of
the blood-stream, and also the reno-pericardial communication. In many
of the Nudibranchiate Opisthobranchs the nervous system presents a
concentration of the ganglia (fig. 48), contrasting greatly with what
we have seen in _Aplysia_. Not only are the pleural ganglia fused to
the cerebral, but also the visceral to these (see in further
illustration the condition attained by the Pulmonate _Limnaeus_, fig.
59), and the visceral loop is astonishingly short and insignificant
(fig. 48, e'). That the parts are rightly thus identified is
probable from J.W. Spengel's observation of the osphradium and its
nerve-supply in these forms; the nerve to that organ, which is placed
somewhat anteriorly--on the dorsal surface--being given off from the
hinder part (visceral) of the right compound ganglion--the fellow to
that marked A in fig. 48. The Eolid-like Nudibranchs, amongst other
specialities of structure, possess (in some cases at any rate)
apertures at the apices of the "cerata" or dorsal papillae, which lead
from the exterior into the hepatic caeca. Some amongst them
(_Tergipes, Eolis_) are also remarkable for possessing peculiarly
modified cells placed in sacs (cnidosacs) at the apices of these same
papillae, which resemble the "thread-cells" of the Coelentera.
According to T.S. Wright and J.H. Grosvenor these nematocysts are
derived from the hydroids on which the animals feed.

The development of many Opisthobranchia has been examined--e.g.
_Aplysia, Pleurobranchidium, Elysia, Polycera, Doris, Tergipes_. All
pass through trochosphere and veliger stages, and in all a nautiloid
or boat-like shell is developed, preceded by a well-marked
"shell-gland" (see fig. 36). The transition from the free-swimming
veliger larva with its nautiloid shell (fig. 36) to the adult form has
not been properly observed, and many interesting points as to the true
nature of folds (whether parapodia or mantle or velum) have yet to be
cleared up by a knowledge of such development in forms like _Tethys,
Doris, Phyllidia_, &c. As in other Molluscan groups, we find even in
closely-allied genera (for instance, in _Aplysia_ and
_Pleurobranchidium_, and other genera), the greatest differences as to
the _amount_ of food-material by which the egg-shell is encumbered.
Some form their diblastula by emboly, others by epiboly; and in the
later history of the further development of the enclosed cells
(arch-enteron) very marked variations occur in closely-allied forms,
due to the influence of a greater or less abundance of food-material
mixed with the protoplasm of the egg.

Sub-order 1.--TECTIBRANCHIA. Opisthobranchs provided in the adult
state with a shell and a mantle, except _Runcina, Pleurobranchaea,
Cymbuliidae_, and some Aplysiomorpha. There is a ctenidium, except in
some Thecosomata and Gymnosomata, and an osphradium.

Tribe 1.--BULLOMORPHA. The shell is usually well developed, except in
_Runcina_ and _Cymbuliidae_, and may be external or internal. No
operculum, except in _Actaeonidae_ and _Limacinidae_. The pallial
cavity is always well developed, and contains the ctenidium, at least
in part; ctenidium, except in _Lophocercidae_, of folded type. With
the exception of the _Aplustridae, Lophocercidae_ and _Thecosomata_,
the head is devoid of tentacles, and its dorsal surface forms a
digging disk or shield. The edges of the foot form parapodia, often
transformed into fins. Posteriorly the mantle forms a large pallial
lobe under the pallial aperture. Stomach generally provided with
chitinous or calcified masticatory plates. Visceral commissure fairly
long, except in _Runcina, Lobiger_ and _Thecosomata_. Hermaphrodite
genital aperture, connected with the penis by a ciliated groove,
except in _Actaeon, Lobiger_ and _Cavolinia longirostris_, in which
the spermiduct is a closed tube. Animals either swim or burrow.

A, _Eolis papillosa_ (Lin.), dorsal view.
a, b, Posterior and anterior cephalic tentacles.
c, The dorsal "cerata."
B, _Tethys leporina_, dorsal view.
a, The cephalic hood.
b, Cephalic tentacles.
c, Neck.
d, Genital pore.
e, Anus.
f, Large cerata.
g, Smaller cerata.
h, Margin of the foot.
C, _Doris (Actinocyclus) tuberculatus_ (Cuv.), seen from the pedal
surface.
m, Mouth.
b, Margin of the head.
f, Sole of the foot.
sp, The mantle-like epipodium.
D, E, Dorsal and lateral view of _Elysia (Actaeon) viridis_.
ep, epipodial outgrowths. (After Keferstein.)]

ph, Pharynx.
m, Midgut, with its hepatic appendages h, all of which are not
figured.
e, Hind gut.
an, Anus.]

A, Cerebral, pleural and visceral ganglia united.
B, Pedal ganglion.
C, Buccal ganglion.
D, Oesophageal ganglion connected with, the Buccal.
a, Nerve to superior cephalic tentacle.
b, Nerves to inferior cephalic tentacles.
c, Nerve to generative organs.
d, Pedal nerve.
e, Pedal commissure.
e', Visceral loop or commissure (?).]

a, Mouth.
b, Pair of cephalic tentacles.
C, C, Pteropodial lobes of the foot.
d, Median web connecting these.
e, e, Processes of the mantle-skirt reflected over the surface of
the shell.
g, The shell enclosing the visceral hump.
h. The median spine of the shell.]

Fam. 1.--_Actaeonidae._ Cephalic shield bifid posteriorly; margins
of foot slightly developed; genital duct diaulic; visceral
commissure streptoneurous; shell thick, with prominent spire and
elongated aperture; a horny operculum. _Actaeon_, British.
_Solidula. Tornatellaea_, extinct. _Adelactaeon. Bullina.
Bullinula._

Fam. 2.--_Ringiculidae._ Cephalic disk enlarged anteriorly, forming
an open tube posteriorly; shell external, thick, with prominent
spire; no operculum. _Ringicula. Pugnus._

Fam. 3.--_Tornatinidae._ Margins of foot not prominent; no radula;
shell external, with inconspicuous spire. _Tornatina_, British.
_Retusa. Volvula._

Fam. 4.--_Scaphandridae._ Cephalic shield short, truncated
posteriorly; eyes deeply embedded; three calcareous stomachal
plates; shell external, with reduced spire. _Scaphander_, British.
_Atys. Smaragdinella. Cylichna_, British. _Amphisphyra_, British.

Fam. 5.--_Bullidae._ Margins of foot well developed; eyes
superficial; three chitinous stomachal plates; shell external, with
reduced spire. Bulla, British. _Haminea_, British.

Fam. 6.--_Aceratidae._ Cephalic shield continuous with neck; twelve
to fourteen stomachal plates; a posterior pallial filament passing
through a notch in shell. _Acera_, British. _Cylindrobulla.
Volutella._

Fam. 7.--_Aplustridae._ Foot very broad; cephalic shield with four
tentacles; shell external, thin, without prominent spire.
_Aplustrum. Hydatina. Micromelo._

Fam. 8.--_Philinidae._ Cephalic shield broad, thick and simple;
shell wholly internal, thin, spire much reduced, aperture very
large. _Philine_, British. _Cryptophthalmus. Chelinodura.
Phanerophthalmus. Colpodaspis_, British. _Colobocephalus._

Fam. 9.--_Doridiidae._ Cephalic shield ending posteriorly in a
median point; shell internal, largely membranous; no radula or
stomachal plates. _Doridium. Navarchus._

Fam. 10.--_Gastropteridae._ Cephalic shield pointed behind; shell
internal, chiefly membranous, with calcified nucleus, nautiloid;
parapodia forming fins. _Gastropteron._

Fam. 11.--_Runcinidae._ Cephalic shield continuous with dorsal
integument; no shell; ctenidium projecting from mantle cavity.
_Runcina._

Fam. 12.--_Lophocercidae._ Shell external, globular or ovoid; foot
elongated, parapodia separate from ventral surface; genital duct
diaulic. _Lobiger. Lophocercus._

f, Postero-dorsal surface.
g, Antero-ventral surface.
h, Median dorsal spine.
i, Mouth of the shell.]

The next three families form the group formerly known as Thecosomatous
Pteropods. They are all pelagic, the foot being entirely transformed
into a pair of anterior fins; eyes are absent, and the nerve centres
are concentrated on the ventral side of the oesophagus.

Fam. 13.--_Limacinidae._ Dextral animals, with shell coiled
pseudo-sinistrally; operculum with sinistral spiral; pallial cavity
dorsal. _Limacina_, British. _Peraclis_, ctenidium present.

Fam. 14.--_Cymbuliidae._ Adult without shell; a sub-epithelial
pseudoconch formed by connective tissue; pallial cavity ventral.
_Cymbulia. Cymbuliopsis. Gleba. Desmopterus._

Fam. 15.--_Cavoliniidae._ Shell not coiled, symmetrical; pallial
cavity ventral. _Cavolinia. Clio. Cuvierina._

Tribe 2.--APLYSIOMORPHA. Shell more or less internal, much reduced or
absent. Head bears two pairs of tentacles. Parapodia separate from
ventral surface, and generally transformed into swimming lobes.
Visceral commissure much shortened, except in _Aplysia_. Genital duct
monaulic; hermaphrodite duct connected with penis by a ciliated
groove. Animals either swim or crawl.

Fam. 1.--_Aplysiidae_. Shell partly or wholly internal, or absent;
foot long, with well-developed ventral surface. _Aplysia. Dolabella.
Dolabrifer. Aplysiella. Phyllaplysia. Notarchus_.

The next six families include the animals formerly known as
Gymnosomatous Pteropods, characterized by the absence of mantle and
shell, the reduction of the ventral surface of the foot, and the
parapodial fins at the anterior end of the body. They are all pelagic.

Fam. 2.--_Pneumonodermatidae_. Pharynx evaginable, with suckers.
_Pneumonoderma. Dexiobranchaea. Spongiobranchaea. Schizobrachium_.

Fam. 3.--_Clionopsidae_. No buccal appendages or suckers; a very
long evaginable proboscis; a quadriradiate terminal branchia.
_Clionopsis_.

Fam. 4.--_Notobranchaeidae_. Posterior branchia triradiate.
Notobranchaea.

Fam. 5.--_Thliptodontidae_. Head very large, not marked off from the
body; neither branchia nor suckers; fins situated near the middle of
the body. _Thliptodon_.

a, Anus.
f, Median portion of the foot.
pn, Pteropodial lobe of the foot.
h, Heart.
i, Intestine.
m. Mouth.
ot, Otocyst.
q, Shell.
r, Nephridium.
s, Oesophagus.
[sigma], Sac containing nutritive yolk.
mb, Mantle-skirt.
mc, Sub-pallial chamber.
Kn, Contractile sinus.]

C, C, The wing-like lobes of the foot.
d, Median fold of same.
e, Copulatory organ.
h, Pointed extremity of the shell.
i, Anterior margin of the shell.
n, Stomach.
o, Liver.
u. Hermaphrodite gonad.]

Fam. 6.--_Clionidae_. No branchia of any kind; a short evaginable
pharynx, bearing paired conical buccal appendages or "cephalocones."
_Clione. Paraclione. Fowlerina_.

Fam. 7.--_Halopsychidae_. No branchia; two long and branched buccal
appendages. _Halopsyche_.

Tribe 3.--PLEUROBRANCHOMORPHA. Two pairs of tentacles. Foot without
parapodia; no pallial cavity, but always a single ctenidium situated
on the right side between mantle and foot. Genital duct diaulic,
without open seminal groove; male and female apertures contiguous.
Visceral commissure short, tendency to concentration of all ganglia in
dorsal side of oesophagus.

Fam. 1.--_Tylodinidae_. Shell external and conical; anterior
tentacles form a frontal veil; ctenidium extending only over right
side; a distinct osphradium. _Tylodina_.

Fam. 2.--_Umbrellidae_. Shell external, conical, much flattened;
anterior tentacles very small, and situated with the mouth in a
notch of the foot below the head; ctenidium very large. _Umbrella_.

Fam. 3.--_Pleurobranchidae_. Shell covered by mantle, or absent;
anterior tentacles form a frontal veil; mantle contains spicules.
_Pleurobranchus. Berthella. Haliotinella. Oscanius_, British.
_Oscaniella. Oscaniopsis. Pleurobranchaea._

Sub-order 2.--NUDIBRANCHIA. Shell absent in the adult; no ctenidium or
osphradium. Body generally slug-like, and externally symmetrical.
Visceral mass not marked off from the foot, except in _Hedylidae._
Dorsal respiratory appendages frequently present. Visceral commissure
reduced; nervous system concentrated on dorsal side of oesophagus.
Marine; generally carnivorous, and brightly coloured, affording many
instances of protective resemblance.

Tribe 1.--TRITONIOMORPHA. Liver wholly or partially contained in the
visceral mass. Anus lateral, on the right side. Usually two rows of
ramified dorsal appendages. Genital duct diaulic; male and female
apertures contiguous.

Fam. 1.--_Tritoniidae._ Anterior tentacles form a frontal veil; foot
rather broad. _Tritonia_, British. _Marionia._

Fam. 2.--_Scyllaeidae._ No anterior tentacles; dorsal appendages
broad and foliaceous; foot very narrow; stomach with horny plates.
_Scyllaea_, pelagic.

Fam. 3.--_Phyllirhoidae._ No anterior tentacles, and no dorsal
appendages; body laterally compressed, transparent; pelagic.
_Phyllirhoë._

Fam. 4.--_Tethyidae._ Head broad, surrounded by a funnel-shaped
velum or hood; no radula; dorsal appendages foliaceous. _Tethys.
Melibe._

Fam. 5.--_Dendronotidae._ Anterior tentacles forming a scalloped
frontal veil; dorsal appendages and tentacles similarly ramified.
_Dendronotus. Campaspe._

Fam. 6.--_Bornellidae._ Dorsum furnished on either side with
papillae, at the base of which are ramified appendages. _Bornella._

Fam. 7.--_Lomanotidae._ Body flattened, the two dorsal borders
prominent and foliaceous. _Lomanotus_, British.

Tribe 2.--DORIDOMORPHA. Body externally symmetrical; anus median,
posterior, and generally dorsal, surrounded by ramified pallial
appendages, constituting a secondary branchia. Liver not ramified in
the integuments. Genital duct triaulic. Spicules present in the
mantle.

a, The mouth.
c, The pteropodial lobes of the foot.
f, The centrally-placed hind-foot.
d, l, e, Three pairs of tentacle-like processes placed at the sides
of the mouth, and developed (in all probability) from the
fore-foot.
o', Anus.
y, Genital pore.
k, Retractor muscles.
o and p, The liver.
u, v, w, Genitalia.]

a, Anus.
br, Secondary branchia surrounding the anus.
t, Cephalic tentacles.

External to the branchia are seen ten club-like processes of the
dorsal wall, these are the "cerata" which are characteristically
developed in another suborder of Opisthobranchs.]

Fam. 1.--_Polyceratidae._ A more or less prominent frontal veil;
branchiae non-retractile. _Euplocamus. Polycera_, British.
_Thecacera_, British. _Aegirus_, British. _Plocamopherus. Palio.
Crimora. Triopa_, British. _Triopella._

Fam. 2.--_Goniodorididae._ Mantle-border projecting; frontal veil
reduced, and often covered by the anterior border of the mantle.
_Goniodoris_, British. _Acanthodoris_, British. _Idalia_, British.
_Ancula_, British. _Doridunculus_. _Lamellidoris. Ancylodoris_,
the only fresh-water Nudibranch, from Lake Baikal.

Fam. 3.--_Heterodorididae_. No branchia. _Heterodoris_.

Fam. 4.--_Dorididae_. Mantle oval, covering the head and the greater
part of the body; anterior tentacles, ill-developed; branchiae
generally retractile. _Doris_, British. _Hexabranchus_.
_Chromodoris_.

Fam. 5.--_Doridopsidae_. Pharynx suctorial; no radula; branchial
rosette on the dorsal surface, above the mantle-border.
_Doridopsis_.

Fam. 6.--_Corambidae_. Anus and branchia posterior, below the
mantle-border. _Corambe_.

Fam. 7.-_-Phyllidiidae_. Pharynx suctorial; branchiae surrounding
the body, between the mantle and foot. _Phyllidia. Fryeria_.

The last three families constitute the sub-tribe Porostomata,
characterized by the reduction of the buccal mass, which is modified
into a suctorial apparatus.

Tribe 3.--EOLIDOMORPHA (_Cladohepatica_). The whole of the liver
contained in the integuments and tegumentary papillae. Genital duct
diaulic; male and female apertures contiguous. The anus is
antero-lateral, except in the _Proctonotidae_, in which it is median.
Tegumentary papillae not ramified, and containing cnidosacs with
nematocysts.

Fam. 1.--_Eolididae_. Dorsal papillae spindle-shaped or club-shaped.
_Eolis_, British. _Facelina_, British. _Tergipes_, British.
_Gonieolis. Cuthona. Embletonia. Galvina. Calma. Hero_.

Fam. 2.--_Glaucidae_. Body furnished with three pairs of lateral
lobes, bearing the tegumentary papillae; foot very narrow; pelagic.
_Glaucus_.

Fam. 3.--_Hedylidae_. Body elongated; visceral mass marked off from
foot posteriorly; dorsal appendages absent, or reduced to a single
pair; spicules in the integument. _Hedyle_.

Fam. 4.--_Pseudovermidae_. Head without tentacles; body elongated;
anus on right side. _Pseudovermis_.

Fam. 5.--_Proctonotidae_. Anus posterior, median; anterior
tentacles, atrophied; foot broad. _Janus_, British. _Proctonotus_,
British.

Fam. 6.--_Dotonidae_. Bases of the rhinophores surrounded by a
sheath; dorsal papillae tuberculated and club-shaped, in a single
row on either side of the dorsum; no cnidosacs. _Doto_, British.
_Gellina. Heromorpha_.

Fam. 7.--_Fionidae_. Dorsal papillae with a membranous expansion;
male and female apertures at some distance from each other; pelagic.
_Fiona_.

Fam. 8.--_Pleurophyllidae_. Anterior tentacles in the form of a
digging shield; mantle without appendages, but respiratory papillae
beneath the mantle-border. _Pleurophyllidia_.

Fam. 9.--_Dermatobranchidae_. Like the last, but wholly without
branchiae. _Dermatobranchus_.

Tribe 4.--ELYSIOMORPHA. Liver ramifies in integuments and extends into
dorsal papillae, but there are no cnidosacs. Genital duct always
triaulic, and male and female apertures distant from each other. No
mandibles, and radula uniserial. Never more than one pair of
tentacles, and these are absent in _Alderia_ and some species of
_Limapontia_.

b, The mouth.
l, The lamelliform sub-pallial gills, which (as in Patella) replace
the typical Molluscan ctenidium.]

Fam. 1.--_Hermaeidae_. Foot narrow; dorsal papillae linear or
fusiform, in several series. _Hermaea_, British. _Stiliger_.
_Alderia_, British.

Fam. 2.--_Phyllobranchidae_. Foot broad; dorsal papillae flattened
and foliaceous. _Phyllobranchus. Cyerce_.

Fam. 3.--_Plakobranchidae_. Body depressed, without dorsal papillae,
but with two very large lateral expansions, with dorsal plications.
_Plakobranchus_.

Fam. 4.--_Elysiidae_. Body elongated, with lateral expansions;
tentacles large; foot narrow. _Elysia_, British. _Tridachia_.

Fam. 5.--_Limapontiidae_. No lateral expansions, and no dorsal
papillae; body planariform; anus dorsal, median and posterior.
_Limapontia_, British. _Actaeonia_, British. _Cenia_.

Order 2 (of the Euthyneura).--PULMONATA. Euthyneurous Gastropoda,
probably derived from ancestral forms similar to the Tectibranchiate
Opisthobranchia by adaptation to a terrestrial life. The ctenidium is
atrophied, and the edge of the mantle-skirt is fused to the dorsal
integument by concrescence, except at one point which forms the
aperture of the mantle-chamber, thus converted into a nearly closed
sac. Air is admitted to this sac for respiratory and hydrostatic
purposes, and it thus becomes a lung. An operculum is present only in
_Amphibola_; a contrast being thus afforded with the operculate
pulmonate Streptoneura (_Cyclostoma_, &c.), which differ in other
essential features of structure from the Pulmonata. The Pulmonata are,
like the other Euthyneura, hermaphrodite, with elaborately developed
copulatory organs and accessory glands. Like other Euthyneura, they
have very numerous small denticles on the lingual ribbon. In aquatic
Pulmonata the osphradium is retained.

In some Pulmonata (snails) the foot is extended at right angles to the
visceral hump, which rises from it in the form of a coil as in
Streptoneura; in others the visceral hump is not elevated, but is
extended with the foot, and the shell is small or absent (slugs).

A, _Helix pomatia_. (From Keferstein.)
B, _Helicophanta brevipes_. (From Keferstein, after Pfeiffer.)
C, _Testacella haliotidea_. (From Keferstein.)
D, _Arion ater_, the great black slug. (From Keferstein.)
a, Shell in A, B, C, shell-sac (closed) in D; b, orifice leading
into the sub-pallial chamber (lung).]

Pulmonata are widely distinguished from a small number of Streptoneura
at one time associated with them on account of their mantle-chamber
being converted, as in Pulmonata, into a lung, and the ctenidium or
branchial plume aborted. The terrestrial Streptoneura (represented in
England by the common genus _Cyclostoma_) have a twisted visceral
nerve-loop, an operculum on the foot, a complex rhipidoglossate or
taenio-glossate radula, and are of distinct sexes. The Pulmonata have
a straight visceral nerve-loop, usually no operculum even in the
embryo, and a multidenticulate radula, the teeth being equi-formal;
and they are hermaphrodite. Some Pulmonata (_Limnaea_, &c.) live in
fresh waters although breathing air. The remarkable discovery has been
made that in deep lakes such _Limnaei_ do not breathe air, but admit
water to the lung-sac and live at the bottom. The lung-sac serves
undoubtedly as a hydrostatic apparatus in the aquatic Pulmonata, as
well as assisting respiration.

The same general range of body-form is shown in Pulmonata as in the
Heteropoda and in the Opisthobranchia; at one extreme we have snails
with coiled visceral hump, at the other cylindrical or flattened slugs
(see fig. 56). Limpet-like forms are also found (fig. 57, _Ancylus_).
The foot is always simple, with its flat crawling surface extending
from end to end, but in the embryo _Limnaea_ it shows a bilobed
character, which leads on to the condition characteristic of
Pteropoda.

The adaptation of the Pulmonata to terrestrial life has entailed
little modification of the internal organization. In one genus
(_Planorbis_) the plasma of the blood is coloured red by haemoglobin,
this being the only instance of the presence of this body in the blood
of Glossophorous Mollusca, though it occurs in corpuscles in the blood
of the bivalves _Arca_ and _Solen_ (Lankester).

[tau], Ovo-testis.
ve, Hermaphrodite duct.
Ed, Albuminiparous gland.
u, Uterine dilatation of the hermaphrodite duct.
d, Digitate accessory glands on the female duct.
ps, Calciferous gland or dart-sac on the female duct.
Rf, Spermatheca or receptacle of the sperm in copulation, opening
into the female duct.
vd, Male duct (vas deferens).
p, Penis.
fl, Flagellum.]

The generative apparatus of the snail (_Helix_) may serve as an
example of the hermaphrodite apparatus common to the Pulmonata and
Opisthobranchia (fig. 58). From the ovo-testis, which lies near the
apex of the visceral coil, a common hermaphrodite duct ve proceeds,
which receives the duct of the compact white albuminiparous gland, Ed,
and then becomes much enlarged, the additional width being due to the
development of glandular folds, which are regarded as forming a uterus
u. Where these folds cease the common duct splits into two portions, a
male and a female. The male duct vd becomes fleshy and muscular near
its termination at the genital pore, forming the penis p. Attached to
it is a diverticulum fl, in which the spermatozoa which have descended
from the ovo-testis are stored and modelled into sperm ropes or
spermatophores. The female portion of the duct is more complex. Soon
after quitting the uterus it is joined by a long duct leading from a
glandular sac, the spermatheca (Rf). In this duct and sac the
spermatophores received in copulation from another snail are lodged.
In _Helix hortensis_ the spermatheca is simple. In other species of
_Helix_ a second duct (as large in _Helix aspersa_ as the chief one)
is given off from the spermathecal duct, and in the natural state is
closely adherent to the wall of the uterus. This second duct has
normally no spermathecal gland at its termination, which is simple and
blunt. But in rare cases in _Helix aspersa_ a second spermatheca is
found at the end of this second duct. Tracing the widening female duct
onwards we now come to the openings of the digitate accessory glands
d, d, which probably assist in the formation of the egg-capsule. Close
to them is the remarkable dart-sac ps, a thick-walled sac, in the
lumen of which a crystalline four-fluted rod or dart consisting of
carbonate of lime is found. It is supposed to act in some way as a
stimulant in copulation, but possibly has to do with the calcareous
covering of the egg-capsule. Other Pulmonata exhibit variations of
secondary importance in the details of this hermaphrodite apparatus.

The nervous system of _Helix_ is not favourable as an example on
account of the fusion of the ganglia to form an almost uniform ring of
nervous matter around the oesophagus. The pond-snail (_Limnaeus_)
furnishes, on the other hand, a very beautiful case of distinct
ganglia and connecting cords (fig. 59). The demonstration which it
affords of the extreme shortening of the Euthyneurous visceral
nerve-loop is most instructive and valuable for comparison with and
explanation of the condition of the nervous centres in Cephalopoda, as
also of some Opisthobranchia. The figure (fig. 59) is sufficiently
described in the letterpress attached to it; the pair of buccal
ganglia joined by the connectives to the cerebrals are, as in most of
our figures, omitted. Here we need only further draw attention to the
osphradium, discovered by Lacaze-Duthiers, and shown by Spengel to
agree in its innervation with that organ in all other Gastropoda. On
account of the shortness of the visceral loop and the proximity of the
right visceral ganglion to the oesophageal nerve-ring, the nerve to
the osphradium and olfactory ganglion is very long. The position of
the osphradium corresponds more or less closely with that of the
vanished right ctenidium, with which it is normally associated. In
_Helix_ and _Limax_ the osphradium has not been described, and
possibly its discovery might clear up the doubts which have been
raised as to the nature of the mantle-chamber of those genera. In
_Planorbis_, which is sinistral (as are a few other genera or
exceptional varieties of various Anisopleurous Gastropods), instead of
being dextral, the osphradium is on the left side, and receives its
nerve from the left visceral ganglion, the whole series of unilateral
organs being reversed. This is, as might be expected, what is found
to be the case in all "reversed" Gastropods.

The shell of the Pulmonata, though always light and delicate, is in
many cases a well-developed spiral "house" into which the creature can
withdraw itself; and, although the foot possesses no operculum, yet in
_Helix_ the aperture of the shell is closed in the winter by a
complete lid, the "hybernaculum" more or less calcareous in nature,
which is secreted by the foot. In _Clausilia_ a peculiar modification
of this lid exists permanently in the adult, attached by an elastic
stalk to the mouth of the shell, and known as the "clausilium." In
_Limnaeus_ the permanent shell is preceded in the embryo by a
well-marked shell-gland or primitive shell-sac (fig. 60), at one time
supposed to be the developing anus, but shown by Lankester to be
identical with the "shell-gland" discovered by him in other Mollusca
(_Pisidium, Pleurobranchidium, Neritina_, &c.). As in other Gastropoda
Anisopleura, this shell-sac may abnormally develop a plug of chitinous
matter, but normally it flattens out and disappears, whilst the
cap-like rudiment of the permanent shell is shed out from the
dome-like surface of the visceral hump, in the centre of which the
shell-sac existed for a brief period.

ce, Cerebral ganglion.
pe, Pedal ganglion.
pl, Pleural ganglion.
ab, Abdominal ganglion.
sp, Visceral ganglion of the left side; opposite to it is the
visceral ganglion of the right side, which gives off the long nerve
to the olfactory ganglion and osphradium o.

In _Planorbis_ and in _Auricula_ (Pulmonata, allied to _Limnaeus_)
the olfactory organ is on the _left_ side and receives its nerve
from the _left_ visceral ganglion. (After Spengel.)]

In _Clausilia_, according to the observations of C. Gegenbaur, the
primitive shell-sac does not flatten out and disappear, but takes the
form of a flattened closed sac. Within this closed sac a plate of
calcareous matter is developed, and after a time the upper wall of the
sac disappears, and the calcareous plate continues to grow as the
nucleus of the permanent shell. In the slug _Testacella_ (fig. 56, C)
the shell-plate never attains a large size, though naked. In other
slugs, namely, _Limax_ and _Arion_, the shell-sac remains permanently
closed over the shell-plate, which in the latter genus consists of a
granular mass of carbonate of lime. The permanence of the primitive
shell-sac in these slugs is a point of considerable interest. It is
clear enough that the sac is of a different origin from that of
_Aplysia_ (described in the section treating of Opisthobranchia),
being primitive instead of secondary. It seems probable that it is
identical with one of the open sacs in which each shell-plate of a
_Chiton_ is formed, and the series of plate-like imbrications which
are placed behind the single shell-sac on the dorsum of the curious
slug, _Plectrophorus_, suggest the possibility of the formation of a
series of shell-sacs on the back of that animal similar to those which
we find in _Chiton_. Whether the closed primitive shell-sac of the
slugs (and with it the transient embryonic shell-gland of all other
Mollusca) is precisely the same thing as the closed sac in which the
calcareous pen or shell of the Cephalopod _Sepia_ and its allies is
formed, is a further question which we shall consider when dealing
with the Cephalopoda. It is important here to note that _Clausilia_
furnishes us with an exceptional instance of the _continuity_ of the
shell or secreted product of the primitive shell-sac with the adult
shell. In most other Mollusca (Anisopleurous Gastropods, Pteropods and
Conchifera) there is a want of such continuity; the primitive
shell-sac contributes no factor to the permanent shell, or only a very
minute knob-like particle (_Neritina_ and _Paludina_). It flattens out
and disappears before the work of forming the permanent shell
commences. And just as there is a break at this stage, so (as observed
by A. Krohn in _Marsenia_ = _Echinospira_) there _may_ be a break at a
later stage, the nautiloid shell formed on the larva being cast, and a
new shell of a different form being formed afresh on the surface of
the visceral hump. It is, then, in this sense that we may speak of
primary, secondary and tertiary shells in Mollusca recognizing the
fact that they _may_ be merely phases fused by continuity of growth so
as to form but one shell, or that in other cases they _may_ be
presented to us as separate individual things, in virtue of the
non-development of the later phases, or in virtue of sudden changes in
the activity of the mantle-surface causing the shedding or
disappearance of one phase of shell-formation before a later one is
entered upon.

The development of the aquatic Pulmonata from the egg offers
considerable facilities for study, and that of _Limnaeus_ has been
elucidated by E.R. Lankester, whilst H. Rabl has with remarkable skill
applied the method of sections to the study of the minute embryos of
_Planorbis_. The chief features in the development of _Limnaeus_ are
exhibited in fig. 60. There is not a very large amount of
food-material present in the egg of this snail, and accordingly the
cells resulting from division are not so unequal as in many other
cases. The four cells first formed are of equal size, and then four
smaller cells are formed by division of these four so as to lie at one
end of the first four (the pole corresponding to that at which the
"directive corpuscles" are extruded and remain). The smaller cells now
divide and spread over the four larger cells; at the same time a
space--the cleavage cavity or blastocoel--forms in the centre of the
mulberry-like mass. Then the large cells recommence the process of
division and sink into the hollow of the sphere, leaving an elongated
groove, the blastopore, on the surface. The invaginated cells (derived
from the division of the four big cells) form the endoderm or
arch-enteron; the outer cells are the ectoderm. The blastopore now
closes along the middle part of its course, which coincides in
position with the future "foot." One end of the blastopore becomes
nearly closed, and an ingrowth of ectoderm takes place around it to
form the stomodaeum or fore-gut and mouth. The other extreme end
closes, but the invaginated endoderm cells remain in continuity with
this extremity of the blastopore, and form the "rectal peduncle" or
"pedicle of invagination" of Lankester, although the endoderm cells
retain no contact with the middle region of the now closed-up
blastopore. The anal opening forms at a late period by a very short
ingrowth or proctodaeum coinciding with the blind termination of the
rectal peduncle (fig. 60, pi).

ph, Pharynx (stomodaeal invagination).
v, v, The ciliated band marking out the velum.
ng, Cerebral nerve-ganglion.
re, Stiebel's canal (left side), probably an evanescent embryonic
nephridium.
sh, The primitive shell-sac or shell-gland.
pi, The rectal peduncle or pedicle of invagination; its attachment
to the ectoderm is coincident with the hindmost extremity of the
elongated blastopore of fig. 3, C.
tge, Mesoblastic (skeleto-trophic and muscular) cells investing
gs, the bilobed arch-enteron or lateral vesicles of invaginated
endoderm, which will develop into liver.
f, The foot.]

The body-cavity and the muscular, fibrous and vascular tissues are
traced partly to two symmetrically disposed "mesoblasts," which bud
off from the invaginated arch-enteron, partly to cells derived from
the ectoderm, which at a very early stage is connected by long
processes with the invaginated endoderm. The external form of the
embryo goes through the same changes as in other Gastropods, and is
not, as was held previously to Lankester's observations, exceptional.
When the middle and hinder regions of the blastopore are closing in,
an equatorial ridge of ciliated cells is formed, converting the embryo
into a typical trochosphere.

The foot now protrudes below the mouth, and the post-oral hemisphere
of the trochosphere grows more rapidly then the anterior or velar
area. The young foot shows a bilobed form. Within the velar area the
eyes and the cephalic tentacles commence to rise up, and on the
surface of the post-oral region is formed a cap-like shell and an
encircling ridge, which gradually increases in prominence and becomes
the freely depending mantle-skirt. The outline of the velar area
becomes strongly emarginated and can be traced through the more mature
embryos to the cephalic lobes or labial processes of the adult
_Limnaeus_ (fig. 61).

The increase of the visceral dome, its spiral twisting, and the
gradual closure of the space overhung by the mantle-skirt so as to
convert it into a lung-sac with a small contractile aperture, belong
to stages in the development later than any represented in our
figures.

We may now revert briefly to the internal organization at a period
when the trochosphere is beginning to show a prominent foot growing
out from the area where the mid-region of the elongated blastopore was
situated, and having therefore at one end of it the mouth and at the
other the anus. Fig. 60 represents such an embryo under slight
compression as seen by transmitted light. The ciliated band of the
left side of the velar area is indicated by a line extending from v to
v; the foot f is seen between the pharynx ph and the pedicle of
invagination pi. The mass of the arch-enteron or invaginated
endodermal sac has taken on a bilobed form, and its cells are swollen
(gs and tge). This bilobed sac becomes _entirely_ the liver in the
adult; the intestine and stomach are formed from the pedicle of
invagination, whilst the pharynx, oesophagus and crop form from the
stomodaeal invagination ph. To the right (in the figure) of the rectal
peduncle is seen the deeply invaginated shell-gland ss, with a
secretion sh protruding from it. The shell-gland is destined in
_Limnaeus_ to become very rapidly stretched out, and to disappear.
Farther up, within the velar area, the rudiments of the cerebral
nerve-ganglion ng are seen separating from the ectoderm. A remarkable
cord of cells having a position just below the integument occurs on
each side of the head. In the figure the cord of the left side is
seen, marked re. This paired organ consists of a string of cells which
are perforated by a duct opening to the exterior and ending internally
in a flame-cell. Such cannulated cells are characteristic of the
nephridia of many worms, and the organs thus formed in the embryo
_Limnaeus_ are embryonic nephridia. The most important fact about them
is that they disappear, and are in no way connected with the typical
nephridium of the adult. In reference to their first observer they
were formerly called "Stiebel's canals." Other Pulmonata possess, when
embryos, Stiebel's canals in a more fully developed state, for
instance, the common slug _Limax_. Here too they disappear during
embryonic life. Similar larval nephridia occur in other Gastropoda. In
the marine Streptoneura they are ectodermic projections which
ultimately fall off; in the Opisthobranchs they are closed pouches; in
_Paludina_ and _Bithynia_ they are canals as in Pulmonata.

_Marine Pulmonata._--Whilst the Pulmonata are essentially a
terrestrial and fresh-water group, there is one genus of slug-like
Pulmonates which frequent the sea-coast (_Oncidium_, fig. 62). Karl
Semper has shown that these slugs have, in addition to the usual pair
of cephalic eyes, a number of eyes developed upon the dorsal
integument. These dorsal eyes are very perfect in elaboration,
possessing lens, retinal nerve-end cells, retinal pigment and optic
nerve. Curiously enough, however, they differ from the cephalic
Molluscan eye in the fact that, as in the vertebrate eye, the
filaments of the optic nerve penetrate the retina, and are connected
with the surfaces of the nerve-end cells nearer the lens instead of
with the opposite end. The significance of this arrangement is not
known, but it is important to note, as shown by V. Henson, S.J.
Hickson and others, that in the bivalves _Pecten_ and _Spondylus_,
which also have eyes upon the mantle quite distinct from typical
cephalic eyes, there is the same relationship as in Oncidiidae of the
optic nerve to the retinal cells. In both Oncidiidae and _Pecten_ the
pallial eyes have probably been developed by the modification of
tentacles, such as coexist in an unmodified form with the eyes. The
Oncidiidae are, according to K. Semper, pursued as food by the leaping
fish _Periophthalmus_, and the dorsal eyes are of especial value to
them in aiding them to escape from this enemy.

Sub-order 1.--BASOMMATOPHORA. Pulmonata with an external shell. The
head bears a single pair of contractile but not invaginable tentacles,
at the base of which are the eyes. Penis at some distance from the
female aperture, except in _Amphibola_ and _Siphonaria_. All have an
osphradium, except the _Auriculidae_, which are terrestrial, and it is
situated outside the pallial cavity in those forms in which water is
not admitted into the lung. There is a veliger stage in development,
but the velum is reduced.

Fam. 1.--_Auriculidae_. Terrestrial and usually littoral; genital
duct monaulic, the penis being connected with the aperture by an
open or closed groove; shell with a prominent spire, the internal
partitions often absorbed and the aperture denticulated. _Auricula.
Cassidula. Alexia. Melampus. Carychium_, terrestrial, British.
_Scarabus. Leuconia_, British. _Blauneria. Pedipes_.

Fam. 2.--_Otinidae_. Shell with short spire, and wide oval aperture;
tentacles short. _Otina_, British. _Camptonyx_, terrestrial.