Protective coloration is not uncommon among the Mollusca. Littorina obtusata is habitually found, on our own coasts, on Fucus vesiculosus, the air-bladders of which it closely resembles in colour and shape. Littorina pagodus, a large and showy species, resembles so closely the spongy crumbling rocks of Timor, on which it lives, that it can hardly be discerned a pace off. Helcion pellucidum, the common British ‘blue limpet,’ lives, when young, almost exclusively on the iridescent leaves of the great Laminariae, with the hues of which its own conspicuous blue lines harmonise exactly. In mature life, when the Helcion invariably transfers its place of abode to the lower parts of the stalk and finally to the root of the Laminaria, which are quite destitute of iridescence, these blue lines disappear or become much less marked.
The specimens of Purpura lapillus which occur at Newquay in Cornwall are banded with rings of colour, especially with black and white, in a more varied and striking way than any other specimens that have ever occurred to my notice. I am inclined to refer this peculiarity to a tendency towards protective coloration, since the rocks on which the Purpura occurs are often banded with veins of white and colour, and variegated to a very marked extent.
Ovula varies the colour of its shell from yellow to red, to match the colour of the Gorgonia on which it lives. The same is the case with Pedicularia, which occurs on red and yellow coral.
Helix desertorum, by its gray-brown colour, harmonises well with the prevailing tint of the desert sands, among which it finds a home. Benson observes that the gaudy H. haemastoma, which lives on the trunks of palm-trees in Ceylon, daubs its shell with its excrement. Our own Buliminus obscurus, which lives principally on the trunks of smooth-barked trees, daubs its shell with mud, and must often escape the observation of its enemies by its striking resemblance to the little knots on the bark, especially of beech trees, its favourite haunt. Some species of Microphysa, from the West Indies, habitually encrust their shells with dirt, and the same peculiarity in Vitrina has already been mentioned. Ariophanta Dohertyi Aldr., a recent discovery from Sumatra, is of a green colour, with a singularly delicate epidermis; it is arboreal in its habits, and is almost invisible amongst the foliage.[155] Many of our own slugs, according to Scharff, are coloured protectively according to their surroundings. A claret-coloured variety of Arion ater occurred to this observer only in pine woods, where it harmonised with the general colouring of the ground and the pine-needles, while young winter forms of the same species choose for hiding-places the yellow fallen leaves, whose colour they closely resemble. Limax marginatus (= arborum Bouch.) haunts tree trunks, and may easily be mistaken for a piece of bark; Amalia carinata lives on and under the ground, and in colour resembles the mould; Arion intermedius feeds almost exclusively on fungi, to which its colour, which is white, gray, or light yellow, tends to approximate it closely; Geomalacus maculosus conceals itself by its striking resemblance to the lichens which grow on the surface of rocks, and actually presumes on this resemblance so much as to expose itself, contrary to the usual custom of its congeners, to the full light of the afternoon sun.[156]
Several views have been advanced with regard to the dorsal papillae, or cerata, in the Nudibranchs. Professor W. A. Herdman, who has examined a considerable number of our own British species, in which these processes occur, is of opinion[157] that they are of two quite distinct kinds. In the first place, they may contain large offshoots, or diverticula, of the liver, and thus be directly concerned in the work of digestion. This is the case with Aeolis and Doto. In the second place, they may be simply lobes on the skin, with no connexion with the liver, and no special function to perform. This is the case with Tritonia, Ancula, and Dendronotus.
Professor Herdman is of opinion that although the cerata may in all cases aid in respiration to a certain extent, yet that extent is so small as to be left out of consideration altogether. He regards the cerata in both the two classes mentioned above as “of primary importance in giving to the animals, by their varied shapes and colours, appearances which are in some cases protective, and in others conspicuous and warning.”
Thus, for instance, Tritonia plebeia, which is fairly abundant at Puffin and Hilbre Is., appears always to be found creeping on the colonies of a particular polyp, Alcyonium digitatum, and nowhere else. The specimens in each colony of the polyp differ noticeably both in the matter of colour, and of size, and of varied degrees of expansion. The Tritonia differs also, being marked in varied tints of yellow, brown, blue, gray, black, and opaque white, in such a way as to harmonise with the varied colours of the Alcyonium upon which it lives. The cerata on the back of the Tritonia contribute to this general resemblance. They are placed just at the right distance apart, and are just the right size and colour, to resemble the crown of tentacles on the half-expanded polyp.
Similarly, Doto coronata, which, when examined by itself, is a very conspicuous animal, with showy, bright-coloured cerata, is found by Professor Herdman to haunt no other situations but the under side of stones and overhanging ledges of rock which are colonised by a hydroid, known as Clava multicornis. The Doto is masked by the tentacles and clusters of sporosacs on the zoophyte, with whose colouring and size its own cerata singularly correspond. A similar and even more deceptive correspondence with environment was noticed in the case of the very conspicuous Dendronotus arborescens.
In these cases, the colouring and general shape of the cerata are protective, i.e. they match their surroundings in such a way as to enable the animal, in all probability, to escape the observation of its enemies. According to Professor Herdman, however, the brilliant and showy coloration of the cerata of Aeolis is not protective but ‘warning.’ Aeolis does not hide itself away as if shunning observation, like Doto, Tritonia, and Dendronotus; on the contrary, it seems perfectly fearless and indifferent to being noticed. Its cerata are provided with sting-cells, like those of Coelenterata, at their tips, and its very conspicuousness is a warning to its enemies that they had better not try to attack it, just as the showy white tail of the skunk acts as a sort of danger-signal to its own particular foes. It is important for the Aeolis, not merely to be an unpalatable nettle in animal shape, but also to be conspicuous enough to prevent its being experimented upon as an article of food, in mistake for something less nasty.
Professor Herdman subsequently conducted some experiments[158] with fishes, with the view of testing his theory that the shapes and colours of Nudibranchs serve the purpose either of protection or warning, and bear direct relation to the creature’s edibility. These experiments, on the whole, distinctly tended to confirm the theory. Aeolis was evidently very nasty, and probably stung the mouths of the fishes who tried it. For the complete success of the theory, they ought to have let it severely alone, but the fish were evidently accustomed to make a dash at anything that was dropped into their tank. Another conspicuous mollusc, Ancula cristata, was introduced, Professor Herdman and his collaborator each commencing operations by eating a live specimen themselves. They found the taste pleasant, distinctly like that of an oyster. The fish, however, when the experiments were conducted under conditions which made the scene as much like ‘real life’ as possible, did not agree with Professor Herdman. The Ancula crawled over various parts of the tank for several days untouched by the fish, who sometimes went close to them and looked at them, but never attempted to taste them. Experiments with species whose colours were protective, such as Dendronotus, were also conducted, and the decided edibility of these species was established, the fish competing eagerly for them, and tearing them rapidly to pieces.