N UUIBUANCHIATA.- 
MOLLUSCA. Tkito.nia. 351 
foot which is adapted for walking, and accordingly they 
are generally found crawling slowly along, either on the 
rocks near low water mark, on the firm rocky bottom at 
no great depth, or clinging to sea-weeds out at sea. They 
do not appear to have the power of swimming through 
the water ; but they are frequently to be seen floating 
on the surface in an inverted position. The species are 
numerous, and are found in all parts of the world. The 
British species have been studied with greater care than 
the exotic, and no fewer than one hundred have been 
described by Messrs. Alder and Hancock as inhabiting 
our own coasts. They exhibit a very great variety of 
form, a high state of organization, and an astonishing 
variety of lively and beautiful colours. The gills being 
contractile into cavities on the surface of the body, they 
present in the living state extremely interesting objects 
of observation, as they keep extruding them and with- 
drawing them at frequent intervals. In some they 
present the appearance of flowers ; in others they are 
arborescent or tree -like, or feathered like an ostrich 
plume; and in others they are disposed in rows of 
papillary tubercles on the sides of the body. The greater 
number of these Sea-slugs are carnivorous, and appear 
to be very voracious. They feed chiefly upon zoophytes 
and sponges; some adding to their bill of fare the 
gelatinous Porpitse and Velellai that are found float- 
ing on the surface of the ocean ; while others again 
have been seen devouring other Nudibranchs, and 
even making a repast upon their own spawn. Messrs. 
Alder and Hancock tell us that large individuals of 
the Eolis coronata feed on their own species ; and 
should a small or weak specimen be within their reach, 
they seize hold of it by any part which may be nearest 
to them. “ The tail, however, is generally first seized, 
and fierce and determined is the onset. The devourer 
raises and shakes his papillae in the manner that the 
porcupine shakes its quills when irritated, and then, 
laying back the dorsal tentacles and curling up the 
oval ones, fixes the protruded mouth and jaws upon 
his prey, when, with a convulsive shrinking up of the 
body, morsel after morsel is appropriated. In this man- 
ner it is not uncommon to see an individual entirely 
devour another, half its own size.” These animals 
deposit their ova in coils, long ribands, or spiral cliains. 
The eggs are very numerous. Mr. Darwin notices 
particularly a species found on the shore of the Falk- 
land Islands. The ova were contained, from two to 
five in number, in a spherical little case ; and these 
were arranged two deep in transverse rows, forming a 
riband about twenty inches in length. P.y computa- 
tion of the number of rows, and the number of little 
cases in each row, he reckoned that in that riband there 
could not be fewer than six hundred thousand eggs ! 
Upon escaping from the ovum the larva, or young, 
is extremely minute, and is described as resembling 
a rotiferous animalcule more than a mollusc. It is 
inclosed in a transparent, calcareous, nautiloid shell, 
with an o[)erculum. In lieu of the tentacles which are 
possessed by the adult, this young creature has two 
veils, shaped somewhat like ears, and which give it the 
power of moving rapidly through the water. At a later 
period these veils disappear ; the foot becomes enlarged 
and the shell falls off ; the creature begins to crawl like 
a Gasteropod, and the gills make their appearance. 
When the veils fall off, and the tentacles and gills are 
fully evolved, the metamorphosis is complete, and the 
animal assumes its mature state of existence. Tlie 
various genera which compose this order may be all 
arranged in two large groups, according to the situation 
of the gills or branchiae. 
Group L— PYGOBRANCHIA {Gray) = K^-viio- 
Branchiata {Adams). 
In this group the gills are plumose or branched, and 
are placed in a circle or semicircle round the vent, on 
the middle of the hinder part of the back. The skin 
is tough and coriaceous, of a spongy or cellular struc- 
ture, and stiffened with numerous imbedded spicula 
more or less definitely arranged. In some of the 
families in this group, as in OnclndoridcB and Doridw, 
the body of the animal is of a convex form, the mantle 
large, reaching to the edge of the foot, and destitute 
of any appendages. The dorsal tentacles are two in 
number, retractile ; and the mouth tentacles are also 
two, when present, but are sometimes wanting. These 
two families are represented on Plate 5 by several 
species. In all the other families of this group the 
body of the animal is more or less compiessed, the 
mantle small, exposing the head and foot, or some- 
times even obsolete, and often furnished on the margin 
with beard-like filaments. Such are the families Gonio- 
doridcB, Polyceratidce, and Ce7'aiosomidce, as represented 
in Plates 5 and 6 by several species. 
Group II.— AlOLOBRANCHIATA. 
The second large group of Nudibranchs are all distin- 
guished from the preceding group, by not having the 
gills arranged round the vent, but on the contrary placed 
usually in rows along the sides of the body. The exact 
position and form of these organs vary, however, a good 
deal. In some (the /«/eTOi?’a72c/ifa of Gray) they are 
placed on the under side of the expanded mouth, and 
are lamellar. Such are the families Phyllidiida: ar.d 
Dipliyllidiidce. In another series of this group (the 
Polyhranchia of Gray) the gills are placed on the 
upper surface of the mantle, and are either lamellar or 
plumose. Such are the families Tritoniidce, ScyllceidcB, 
and TethyidcR. 
Genus Tritonia. — The Tritonias are rather nume- 
rous in species, and many of them are found upon 
corallines or sea-weed, and under stones, in shallow 
water, in which places they feed upon the Zoophytes 
that live in their neighbourhood. — See Plate 6, fig. 1 
{Tritonia Plomhergii). 
THE TRITONIA ARBOEESCENS is remarkable for 
the sound it produces. When observed in a glass vessel 
in a room, the sounds are audible at the distance of 
twelve feet, and resemble very much the clink of a steel 
wire on the side of the jar ; only one stroke is given at 
a time, and this is repeated at intervals of a minute or 
two. “ The sounds obviously proceed from the mouth 
of the animal ; and at the instant of the stroke we ob- 
served the lips suddenly separate, as if to allow the water 
to rush into a small vacuum formed within.” — {Grant.) 
