Mr. W. Garstang, of the Plymouth Laboratory of the Marine Biological Association, confirms[159] Professor Herdman’s views as to the shape and colour of Opisthobranchs. Pleurobranchus membranaceus is known to secrete, on the surface of the body, an acid which reddens blue litmus paper. It is, therefore, no doubt distasteful to fish, which all abominate the taste of acids, and is conspicuously marked with red-brown and yellowish ‘warning’ colours. Haminea and Philine, on the other hand, are good to eat, and consequently possess ‘protective’ coloration. Runcina Hancocki, which is of a brown colour, crawls over brown mud and weeds, but avoids green weeds, on whose surface it would appear conspicuous. Elysia viridis varies its colour according to its habitat, being green when on green weeds, and dark olive, brown, or reddish brown, on pools among tufts of littoral algae. Green specimens of Hermaea dendritica were kept in captivity, and placed in a dish with green and red sea-weeds. They were never observed crawling upon the red weed, upon which they would have been very conspicuous. Archidoris flammea occurred on bright red sponges, to which its colour was so closely assimilated that Mr. Garstang at first quite overlooked it. Goniodoris castanea was found under stones, feeding on compound Ascidians (Botryllus), which it sufficiently resembled to be very inconspicuous in that position.
Again, Jorunna Johnstoni lives[160] upon stones on our southern coast, associated with a certain sponge (Halichondria sp.), which it resembles so closely in outline, in colour, in character of surface, and in its projecting plumes, as to make it very difficult even for the careful observer to distinguish the one from the other. And, since fishes, are known to be distinctly averse to sponges of any kind as an article of food, this resemblance must be decidedly to the advantage of the Jorunna. Another Nudibranch (Calma glaucoides A. and H.) imitates the ova of certain fishes, on which it feeds. Its elongated and depressed form of body, transparent integuments, and silvery gray papillae combine to give it a strong resemblance to the spawn of the fish, which is deposited on stones, the roots of Laminaria, etc.[161]
The common Lamellaria perspicua appears to possess the power of protectively assimilating its colour, markings, etc., to the Ascidians on which it lives. A recent case, occurring off the Isle of Man, is thus described by Professor Herdman.[162] “The mollusc was on a colony of Leptoclinum maculatum, in which it had eaten a large hole. It lay in this cavity so as to be flush with the general surface; and its dorsal integument was not only whitish with small darker marks which exactly reproduced the appearance of the Leptoclinum surface with the ascidiozooids scattered over it, but there were also two larger elliptical clear marks which looked like the large common cloacal apertures of the Ascidian colony.... Presumably the Lamellaria escapes the observation of its enemies through being mistaken for part of the Leptoclinum colony; and the Leptoclinum, being crowded like a sponge with minute sharp-pointed spicules, is, I suppose, avoided as inedible (if not actually noxious through some peculiar smell or taste) by carnivorous animals which might devour such things as the soft unprotected mollusc.”
Parasitic Mollusca
Various grades of parasitism occur among the Mollusca, from the true parasite, living and nourishing itself on the tissues and secretions of its host, to simple cases of commensalism. Some authors have divided these forms into endo- and ectoparasites, according as they live inside or outside of their host. Such a division, however, cannot be rigidly carried out, for certain forms are indifferently endo- and ecto-parasitical, while others are ecto-parasitic in the young form, and become endo-parasitic in the adult. It will be convenient, therefore, simply to group the different forms according to the home on which they find a lodgment.
Fig. 29.—Magilus antiquus L.: A, the adult, imbedded in coral, which has been broken away to show the tube; B, the young (free) form.
On Sponges.—Vulsella and Crenatula almost invariably occur in large masses of irregular shape, boring into sponges. They are especially abundant on Porifera from the Red Sea. Corals form a favourite home of many species, amongst which are several forms of Coralliophila, Rhizochilus, Leptoconchus, and Sistrum. Rhizochilus is a very singular creature, inhabiting branching corals. When adult, it forms irregular shelly extensions of both the inner and outer lips, which adhere to the shafts of the coral, or to the surface of neighbouring shells; at length the aperture becomes completely closed with the exception of the siphonal tube, which becomes long, and consists of the same shelly material. The common Magilus (Fig. [29]), from the Red Sea and Indian Ocean, in the young form is shaped like a small Buccinum. As the coral (Meandrina) to which it attaches itself grows, the Magilus develops at the mouth a long calcareous tube, the aperture of which keeps pace with the growth of the coral, and prevents the mollusc from being entombed. The animal lives at the free, or outer, end of the tube, and is thus continually shifting its position, while the space it abandons becomes completely closed by a mass of solid calcareous matter. Certain species of Ovula inhabit Gorgonia, assuming the colour, yellow or red, of their host, and, in certain cases, developing, probably for prehensile purposes, a pointed extension of the two extremities of the shell. Pedicularia, a form akin to Cypraea, but with a more patulous mouth, inhabits the common Corallium rubrum of the Mediterranean, and another species has been noticed by Graeffe[163] on Melithaea ochracea in Fiji.
On Echinodermata.—(a) Crinoidea. Stylina comatulicola lives on Comatula mediterranea, fixed to the outer skin, which it penetrates by a very long proboscis; the shell is quite transparent.[164] A curious case of a fossil parasite has been noticed by Roberts.[165] A Calyptrea-shaped shell named Platyceras always occurred on the ventral side of a crinoid, encompassed by the arms. For some time this was thought to afford conclusive proof of the rapacity and carnivorous habits of the echinoderm, which had died in the act of seizing its prey. Subsequent investigations, however, showed that in all the cases noticed (about 150) the Platyceras covered the anal opening of the crinoid in such a way that the mouth of the mollusc must have been directly over the orifice of the anus. (b) Asteroidea. The comparatively soft texture of the skin of the starfishes renders them a favourite home of various parasites. The brothers Sarasin noticed[166] a species of Stilifer encysted on the rays of Linckia multiformis. Each shell was enveloped up to the apex, which just projected from a hole at the top of the cyst. The proboscis was long, and at its base was a kind of false mantle, which appeared to possess a pumping action. On the under side of the rays of the same starfish occurred a capuliform mollusc (Thyca ectoconcha), furnished with a muscular plate, whose cuticular surface was indented in such a way as to grip the skin of the Linckia. This plate was furnished with a hole, through which the pharynx projected into the texture of the starfish, acting as a proboscis and apparently furnished with a kind of pumping or sucking action. Adams and Reeve[167] describe Pileopsis astericola as living ‘on the tubercle of a starfish,’ and Stilifer astericola, from the coast of Borneo, as ‘living in the body of a starfish.’ In the British Museum there is a specimen of Pileopsis crystallina ‘in situ’ on the ray of a starfish, (c) On the brittle starfishes (Ophiuroidea) occur several species of Stiliferina. (d) Echinoidea. Various species of Stilifer occur on the ventral spines of echinoids, where they probably subsist on the excreta, and are sometimes found imbedded in the spines themselves. St. Turtoni occurs on the British coasts on several species of Echinus, and Montacuta substriata frequents Spatangus purpureus and certain species of Echinocardium, Cidaris, and Brissus. Lepton parasiticum has been described from Kerguelen I. on a Hemiaster, and a new genus, Robillardia, has recently been established[168] for a Hyalinia-shaped shell, parasitic on an Echinus from Mauritius. (e) Holothurioidea. The ‘sea-cucumbers’ afford lodgment to a variety of curious forms, some of which have experienced such modifications that their generic position is by no means established. Entoconcha occurs fixed by its buccal end to the blood-vessels of certain Synapta in the Mediterranean and the Philippines. Entocolax has been dredged from 180 fath. in Behring’s Straits, attached by its head to certain anterior muscles of a Myriotrochus.[169] A curious case of parasitism is described by Voeltzkow[170] as occurring on a Synapta found between tide-marks on the I. of Zanzibar. In the oesophagus of the Synapta was found a small bivalve (Entovalva), the animal of which was very large for its shell, and almost entirely enveloped the valves by its mantle. As many as five specimens occurred on a single Synapta. In the gut of the same Holothurian lived a small univalve, not creeping freely, but fixed to a portion of the stomach wall by a very long proboscis which pierced through it into the body cavity. This proboscis was nearly three times as long as the animal, and the forward portion of it was set with sharp thorns, no doubt in order to enable it to retain its hold and resist evacuation. Various species of Eulima have been noticed in every part of the world, from Norway to the Philippines, both inside and outside Holothurians.[171] Stilifer also occurs on this section of Echinoderms.[172]
On Annelida.—Cochliolepis parasiticus has been noticed under the scales of Acoetes lupina (a kind of ‘sea-mouse’) in Charleston Harbour.[173]