Internally the buccal parts are characteristic. Above the buccal mass-is a spherical or
elongate muscular organ called the ingluvies buccalis or buccal crop. I t is not known
with any certainty how this organ (which is apparently suctional) acts, but its presence is
nearly always accompanied by a great reduction in the width of the radula,1 which
presents a complete series of gradations. Alciodoris (one species from the northern
Pacific) has no labial armature and has a radula consisting of two large hamate laterals
with twelve or thirteen smaller teeth on either side. In Adalaria the arrangement is
similar, but there is only one hamate tooth. In Acanthodoris there is a labial armature,
and the radula is further reduced in breadth, not exceeding 8 + 1. 0. 1 + 8 in any
species and often being much less. In Lamellidoris the labial cuticle is strengthened with
rings and papillae and the radula i s l + l + l + - l + l o r l + l. 0. 1 + 1. The median
tooth when it exists is small. The first lateral is very large and hamate, the second
merely a small plate.
All the genera of the Pseudodorididae are closely allied to one another and are
remarkable for being restricted to temperate seas. An interesting genus is Galycidoris2
Abraham, in which the branchiae are set in a depression, not amounting to a regular
pocket. This arrangement seems intermediate between the Crypto- and Phanero-
branchiatae. Another remarkable form apparently referable to this group (or possibly , to
the Goniodorididae) is Ancylodoris baicalensis,8 a doridiform mollusc found in Lake Baikal,
I t is the only known example of a Nudibranch inhabiting fresh water, and appears to
have approximated in some respects to the organization of the Prosobranchiata.
The Goniodorididae are nearly allied to the Pseudodorididae, but in external appearance
resemble the Polyceridae. Their buccal parts resemble those of the last family, but the
radula is never more than 1 + 1.0.1 + 1, and in one genus, Drepania, sinks to 1.0.1.
There is always a labial armature; oral tentacles are present, and the rhinophores are not
retractile. This last feature is rare in the Holohepatica, and outside this family is known
to occur only in B'athydoris4 and a few Polycerids. It would be natural to regard it as
primitive, but in some species of Doridopsis normally retractile rhinophores become non-
retractile, and since it is clear that the buccal parts of the Goniodorididae are highly
specialized, the structure of their rhinophores may be secondarily acquired and due to
disuse of the retractile powers.
There are about four genera. Goniodoris is somewhat elongate and bears ridges. In
Idalia the mantle-margin is marked by a line of cirri. In Ancula and Drepania both the
rhinophores and the branchiae are protected by unbranched appendages, which compensate
for the loss of pockets.
The Dorididae Cryptobranchiatae are, both in species and individuals, the most numerous
group of Nudibranchs. More than fifty genera have been described, and some of them
(such as Platydoris, Discodoris, and especially Ghromodoris) have a great number of species.
The common characteristic of all these forms is a branchial rosette completely retractile 1 * 4
1 I t is also found in Corambid® (radula 4 + 1.0.1 ^ 4 ) and various forms with an asco-
glossan radula, such as Phyllobranch/us, PiacoBranchus, Lobiger and Lophocercus.
8 See Eliot, Proc. Malac. Soc., 1907, pp. 359—360.
8 See Dybowski, Deutsch. Malac. Gesellsch., 1900, pp. 143—152.
4 See Eliot, National Antarc. Exped., Nat. Hist., vol. ii, Nudibr., p. 12.
into a permanent pocket. The rhinophores are invariably perfoliate and retractile. True
mandibles are never present; but the labial cuticle is often armed with plates or a ring
formed of hard elements. The radula is nearly always broad, and has rarely (Gadlina,
Tyrinna) a central tooth. The teeth are not much differentiated. Occasionally (Ealla,
Thorunna, Sphserodoris) the first is larger and differently formed. A slighter differentiation
is found in Ghromodoris and some allied genera where the first tooth js rather broader
than the rest and denticulate on both sides. The outermost teeth are often small and imperfectly
formed; they are sometimes denticulate or divided into a bush of long hair-like
spines. Occasionally (Geitodoris, Rostanga, Jorv/n/na) a considerable number of teeth in
the outer part of the rows are differentiated from the rest by being longer and thinner.
Usually the teeth are simple hooks, all alike. More rarely (e. g. Aldisa, Gadlina, and especially
the Chromodorididae) all or some of them are denticulate. There is either a separate
stomach outside thé liver or else the liver-cavity acts as a stomach. There is always a
blood-gland, and it is generally double. The hermaphrodite gland is a layer covering the
liver, except in Alloiodoids, where it is separate. The vas deferens and verge are often
armed with spines, which are sometimes very large and strong as in Platydoris; more
rarely (Kentrodoris, Peronodoris) the verge terminates in a single stylet. There is sometimes
a prostate on the upper part of the vas deferens, and various accessory glands may
occur on both the male and female branches. The general shape is usually flattish and
oblong with a mantle-margin of moderate or considerable width, but Ghromodoris is more
elongate with a narrow margin. The back may be smooth, granulate, villous, or covered
with large or small tubercles of many kinds. Occasionally it is divided into areas by one
or more ridges, and very rarely {Echinodoi'is) it bears tall papillae.
The subdivision of this lfferge class is a matter of great difficulty. The Chromodorididae
(including Ghromodoris, Gasella, Geratosoma, Thorunna* but not, I think, Aphelo-
doris, with which Bergh unites them) form a natural group and so do the Cadlinidae and
perhaps the Kentrodorididas. Some of the other genera, such as Platydoris and Asteronotus,
offer distinct types. But in general it must be confessed that there is little certainty with
regard to either the genera or groups of genera. As a rule the genera were first defined
very precisely from a single species. But when it was subsequently found that other
species did not quite conform to this diagnosis the definition was from time to time
enlarged. The result is that the genera, originally too narrow, end by being too elastic,
or that by modification of the original definition several genera become equivalent. Nor
is this altogether the fault of the classification. The characters of which use can be made
are not very numerous or important, and when the group has been thoroughly examined
it will probably be found that they occur in almost all possible combinations. Also some
of the characters are of doubtful value and consistency. Thus a labial armature is made
by Bergh a generic and even a family characteristic (for the Discodorididae). Yet it is
sometimes allowed to be present or absent in the same genus, for it is found in Platydoris
variegata Bergh and PI. tabulata (Abraham) though absent in the other species. Again,
Diaulula, is separated from Gargamella simply by the character that the latter has an
armature of spines on the male genitalia. But this armature is sometimes absent and
sometimes present in animals ascribed to the same species, viz. Acanthodoris pilosa. I will
not now discuss the general classification of the Cryptobranchiatae further, for it can be