VII.
CLASSIFICATION.
T he class Gastropoda, which is one of the principal divisions of the Mollusca,
contains the two sub-classes Streptoneura and Euthyneura. The former are characterized
by the torsion of the visceral mass and visceral commissure; they are nearly always
dioecious and have only one pair of tentacles. The Euthyneura, on the other hand, are
characterized by the detorsion of the visceral mass and commissure, accompanied by a
marked tendency to concentration of the nervous system; they are without exception
hermaphrodite and generally have two pairs of tentacles. They are divided into two
orders—the Opisthobranchiata,1 with aquatic respiration, and the Pulmonata (snails, slugs,
etc.), which breathe air by means of a pallial cavity. The Opisthobranchiata are
subdivided into the two sub-orders-—Tectibranchiata2 and Nudibranchiata. Those pelagic
Mollusca which have sometimes been treated as a single group under the name of
Pteropods, and sometimes as two groups, the Thecosomata and Gymnosomata, are now
generally classified as families of the Tectibranchiata allied to the Bullidse and Aplysiidse
respectively.
The Opisthobranchiata show such variety in appearance and structure that only a
few general characters can be found applicable to the whole order. They are marine
hermaphrodite Mollusca in which the ventricle is anterior to the auricle. They exhibit a
tendency, which becomes very marked in the more specialized forms, towards external
symmetry, reduction or suppression of the shell, and concentration of the nervous system
round the oesophagus.3
The Nudibranchiata may be defined as marine hermaphrodite Opisthobranchiata
without ctenidium and osphradium, and in the adult state, without a shell. The larva,
however, has a shell and operculum. The visceral mass is not marked off from the foot
(except in the Hedylidse); the body shows complete or approximate external symmetry
and usually bears plumes or other appendages which assist respiration. The nervous
system is concentrated in a collar behind the buccal bulb, and the chief ganglia are placed
on the dorsal surface of the oesophagus, being often partially united and sometimes fused
1 Or Opisthobranchia.
2 Also called Tectibranchia and Nudibranchia. Opisthobranch is strictly speaking opposed to
Prosobranch, which is often used as equivalent to Streptoneura. But whereas the Streptoneura are
almost without exception prosobranch, the word opisthobranch is not applicable to the Pulmonata
and therefore not to the Euthyneura as a whole.
3 Acteon does not conform to this diagnosis for it is streptoneurous and not an Opisthobranch in
the literal sense of the words. But it is generally regarded as the most archaic of the Opisthobranchiata
and allied to the Bullacea in virtue of its cephalic shield, buccal parts, and hermaphrodite
genitalia.
into a single mass. Tlie vas deferens is always an internal tube, never an external
groove. Among other common but not universal characteristics of the Nudibranchiata
are also the following : (I) The dorsal tentacles or rhinophores. are often laminated and
retractile, features not recorded in any other group. (2) The kidney is rarely compact,
but usually a system of ramified; tubes, B The genitalia are often extremely complicated,
both in their essential plan and also owing to the presence of accessory glands and
armatures. Besides this, the various sub-divisions show remarkable peculiarities of their
own, such as the ramification of the digestive organs, the reduction of the radula to a
single row, and the presence of nematocysts.
The Tectibrancbiata are less easily defined. They show a tendency towards the
features presented by Nudihranchs, but as a rule do not possess them in a complete form,
or, at any rate, not all of them together. They often have a pallial cavity and external
seminal groove, with a few exceptions they have, at least, a rudimentary shell, and with
still fewer a well-developed otenidium on the right side. I t is the presence or absence of
this latter and not of the shell which really marks, the distinction between Tectibranchs
and Nudihranchs. In the first the larva grows; over its shell, and, even if the shell
is totally absorbed, develops a ctenidium on the right side where a normal shell
would have protected it, and this asymmetrical position of the ctenidium entails some
asymmetry in the viscera. But in the Nudibranchiata the larva breaks its shell and
grows without reference to the position of a possible sheü igifleyelops, not a otenidium,
but symmetrically disposed gillsiôf various kiSMand the whole structure, both external
and internal, tends-to become symmetrical. The only exceptions to the rule that the
Tectibranchiata have a ctenidium are found among the Pteropods. Not only is the
ctenidium often wanting in these families, hut some genera (Pnewmoderma, Spongio-
branchæa, Glionopsis, Notobrcmehæa) have a posterior breathing organ homologous to the
gills of Nudibrancbs.
It may be asked if these forms should not be included among the Nudibranchiata.
They certainly fulfil nearly all the conditions of the definition, the only exception being
that they have an external seminal groove, but even apart from this groove the structuré
of their foot, parapodia, and head-parts separates them from the Nudibranchiata, and
nothing would be gained by associating them more intimately with that group. But for
all that, they indicate how easily the line between Tectibranchs and Nudibranchs may be
crossed.
The Oncidiidæ, a family of littoral pulmonates represented on British coasts by
OncidieUa, are also Nudibranchs according to the definition, and some forms (e. g. Oncidium
savignyi) bear ramose processes on the back, which doubtless assist respiration. But the
a.nimalfl have also a pulmonary cavity, and a consideration of their whole anatomy leaves
no doubt that they must be classed with the Pulmonata rather than with the Nudibranchiata.
.
A more debatable question as to the extent of the Nudibranchiata is raised by the
Ascoglossa,1 which in the opinion of some eminent authorities (Bergh, von Jhering, Brüel)
form a separate group, characterized chiefly by the possession of a uniseriate radula in which
the teeth have a special form, and do not drop off when worn out but are preserved in a sac
1 Or Saccoglossa-.