on Alcijonium; Solids on Hydroids, Actinia, and sometimes on small molluscs; the
Ascoglossa on the juices of seaweeds. Though our information as to feeding-habits is
not so full as it might be, it seems clear that the various types of buccal parts are adapted
to the various kinds of food, and this is confirmed by the fact that an unusual structure
of these organs is often associated with an unusual diet.1 Thus Dorids have no true
mandibles (though the labial cuticle is sometimes strengthened by a chitinous armature)
but a broad radula. This apparatus is clearly suited to eating a stationary object which
does not need to be captured, and offers no projections to be bitten off, but does require
to be torn to shreds previous to digestion. This function is discharged by the broad
radula which (as shown by the contents of the stomach of D. tuberculata.) converts a piece
of sponge into fragments varying from 0‘5 mm. to 1*5 mm. in length. As the mouth of
Doiis when eating is always hidden by the folds of the foot and mantle, it is impossible to
say how it detaches morsels from the surface of the sponge. I t may be that it everts the
radula8 until it is able to scrape the surface of the whole mass; or it may be that by
suction it is able to draw a portion of sponge into the buccal cavity and triturate it there.
Some species of the genus Doridopsis (which has no radula at all) eat compound Ascidians,
and this seems to show that a Nudibranch has no difficulty in drawing up into its mouth
portions of a relatively soft substance. Nearly all the Cladohepatica3 have mandibles with
cutting edges which act like a pair of shears. Tritonia has likewise a wide radula, which
is natural, since its food, Alcyonium, is spiculous like the sponges eaten by Doris. A chunk
is cut out by the mandibles and then torn to pieces on the radula. But the prey of the
JEolids is «mall and soft. After a piece has been nipped off by the mandibles a single
row of teeth furnished with denticles is sufficient to prepare it for digestion. I he
Ascoglossa have no jaws but a special type of radula (in which only a few teeth, sometimes
not moré than three to four, are in action at the same time) and sometimes a special pouch
termed ingluvies buccalis attached to the buccal mass. Some species (perhaps most) feed
by scratching the surface of seaweeds and sucking the juice up into the ingluvies buccalis
so that the radula is not a masticatory but a penetrating organ capable of breaking up cells
and setting their liquid contents free. The muscular walls of the ingluvies then contract
and force the fluid into the narrow and complicated canals which form the digestive
system. Very exceptional are the feeding habits of Melibe and probably also of Tethys,
an allied European form which I have not had the same opportunities of studying.
Melibe has no radula and only feeble jaws, but its mouth is surrounded externally with a
large funnel which may be more than an inch deep. The margin is contractile and
furnished with long cirri, so that it can either form a wide-open expansion or contract
to a small opening which can be closed by the cirri. In its expanded state this funnel
is used as a net, with which the Melibe sweeps the surface of stones, capturing small|
1 E. g. in Calma glaucoides, which feeds on fish eggs, and Melibe, which captures Crustacea.
2 This of course assumes that the radula can be advanced greatly in front of the position which
it usually occupies. But this is not improbable. The genitalia can be extended to a very great
extent in copulation and oviposition, and the mouth parts may have the same power.
3 The only exceptions are the Ascoglossa, Tethys, and the ambiguous Eedyle.
* See Briiel 1, p. 88, ft.
5 I have never been able to discover how the ingluvies buccalis and peculiar radula act in
Acanthodoris, LamelUdoris, Goniodoris, etc.
crustaceans and even those of relatively considerable size. When any are secured it
tosses the funnel upwards, the margin contracts and the cirri close the opening over the
prey, which is drawn down the short oesophagus into the stomach. Here its hard
integuments are broken up by a girdle of stomach plates. No other Nudibranch is known
to show such an appearance of design in hunting, but Facelina coronata is very voracious
and devours Elysia viridis as well as small Solids, including its own species. Molidia
papillosa attacks and eats sea-anemones,1 a task which demands immunity to their
stinging cells rather than agility. I t usually gnaws the base or column and leaves the
tentacles alone.
The Mollusca are essentially soft animals. None of them, with the exception of the
Cephalopods, are well equipped for a combat or capable of rapid movement. -They can,
in fact, neither fight nor run away. I t is therefore natural to find that in most families
they are protected by a strong shell, safe within which they are not exposed to a higher
percentage of accidents than other marine creatures. We might expect the shell to be
indispensable, but it clearly is not, and some members of most groups and nearly all the
Opisthobranchiata show a decided tendency to reduce its dimensions or get rid of it altogether.
I t is not surprising if this loss of armour is compensated by other methods of
defence manifested both in habits and structure. Soft flexible animals can burrow in mud
or sand and frequent the under-side of stones or thick tufts of seaweed more easily than
those protected by heavy shells. Their mobile and often pellucid integuments are peculiarly
susceptible of changes in shape and colour. This leads to frequent cases of protective
resemblance, that is to say, Nudibranchs escape notice because their colour and to some
extent their shape fit in with their surroundings. Of the reality of this phenomenon no
one can doubt who has collected them in the tropics. , Dorids several inches long remain
invisible on the surface of a stone surrounded by seaweed, and are detected only by touch.
Brilliantly red and yellow species frequent Ascidians and sponges of the same colour, among
which they attract no notice. Similarly, among British species the two red Dorids
llostanga coccinea and Doris flammea live on red sponges (the former on Microciona
atrasanguinea), and Jorunna johnstoni can hardly be detected on Halichondria panicea,
where it is usually found. Galina glaucoides eludes observation among the eggs of fish,
which it eats, and many forms bearing prominences or branched processes, such as
AEgires, Doto and Dendronolus, have been noticed to correspond exactly in colour with the
Bryozoa or Algae, which form their usual habitat. Hermasa bifida is an interesting
example of such correspondence. It lives on red seaweeds of the genus Grifiithsia, with
which the general outline of its body does not harmonize. But in the water this outline
is indefinite; the integuments and most of the organs are transparent and colourless, so
that the only conspicuous features are the red hepatic canals in the body and their red
diverticula in the cerata. These exactly imitate the fine branches of the seaweed in form
and colour, the latter indeed being transferred from the plant to the animal which sucks
its juices.
These resemblances are striking and clearly cannot be disadvantageous to the molluscs,
but still I think that much of the language used about protective coloration, as if the animals
were made to look like their surroundings by some special power either in themselves or
1 Chiefly Actinia and Anthea. I t seems to he afraid of Tealia.