On Crustacea.—A mussel, ⅜ in. long, has been found[174] living under the carapace of the common shore-crab (Carcinus maenas), and one case has been noticed[175] where two mussels, one of several months’ growth, the other smaller, well secured by their byssi, were found under the abdomen of the same species, in such a position as to force the appendages apart and askew. These, however, are not so much cases of parasitism as of involuntary habitat, the mussel no doubt having become involved in the branchiae and the abdomen of the crab in the larval form.
On Mollusca.—A species of Odostomia (pallida Mont.) is found on our own coasts on the ‘ears’ of Pecten maximus, and also[176] on the operculum of Turritella communis. Another species (O. rissoides) frequently occurs in hiding under beds of mussels, but it is not clear whether the habitat is due to parasitism, or simply to the fact that the mass of mussels, knitted together and to the rock by the byssi, affords the Odostomia a safe lurking-place. At Panama the present writer found Crepidula (2 sp.) plentiful on the opercula of the great Strombus galea and of Cerithium irroratum. In each case the parasite exactly fitted the size of the operculum, and had assumed its colour, dark brown or chestnut. Amalthea is very commonly found on Conus, Turbo, and other large shells from the South Pacific, but this is probably not a case of parasitism, but simply of convenience of habitat, just as young oysters are frequently seen on the carapace and even on the legs of large crabs.
Fig. 30.—Crepidula onyx Sowb., parasitic on the operculum of Strombus galeatus Swains., Panama.
On Tunicata.—Lamellaria deposits its eggs and lives on an Ascidian (Leptoclinum), and the common Modiolaria marmorata lives in colonies imbedded in the test of Ascidia mentula and other simple Ascidians.
Fig. 31.—Two species of Eulima: A is sessile on the skin of a Holothurian, through which it plunges its sucking proboscis (Pr); B creeps freely in the stomach of a Holothurian. (After K. Semper.)
Special points of interest with regard to parasitic Mollusca relate to (1) Colour. This is in most cases absent, the shell being of a uniform hyaline or milky white. This may be due, in the case of the endo-parasitic forms, to absence of light, and possibly, in those living outside their host, to some deficiency in the nutritive material. A colourless shell is not necessarily protective, for though a transparent shell might evade detection, a milk-white hue would probably be conspicuous. (2) Modifications of structure. These are in many cases considerable. Entoconcha and Entocolax have no respiratory or circulatory organs, and no known nervous system; Thyca and certain Stilifer possess a curious suctorial apparatus; the foot in many cases has aborted, since the necessity for locomotion is reduced to a minimum, and its place is supplied by an enormous development of the proboscis, which enables the creature to provide itself with nutriment without shifting its position. K. Semper notices a case where a Eulima, whose habitat is the stomach of a Holothurian, retains the foot unmodified, while a species occurring on the outer skin, but provided with a long proboscis, has lost its foot altogether.[177] Special provision for holding on is noticed in certain cases, reminding us of similar provision in human parasites. Eyes are frequently, but not always wanting, even in endo-parasitic forms. A specially interesting modification of structure occurs in (3) the Radula or ribbon-shaped arrangement of the teeth. In most cases of parasitism (Eulima, Stilifer, Odostomia, Entoconcha, Entocolax, Magilus, Coralliophila, Leptoconcha) it is absent altogether. In Ovula and Pedicularia, genera which are in all other respects closely allied to Cypraea, the radula exhibits marked differences from the typical radula of the Cypraeidae. The formula (3·1·3) remains the same, but the laterals are greatly produced and become fimbriated, sometimes at the extremity only, sometimes along the whole length. A very similar modification occurs in the radula of Sistrum spectrum Reeve, a species which is known to live parasitically on one of the branching corals. Here the laterals differ from those of the typical Purpuridae in being very long and curved at the extremity. The general effect of these modifications appears to be the production of a radula rather of the type of the vegetable-feeding Trochidae, which may perhaps be regarded as a link in the chain of gradually-degraded forms which eventually terminate in the absence of the organ altogether. The softer the food, the less necessity there is for strong teeth to tear it; the teeth either become smaller and more numerous, or else longer and more slender, and eventually pass away altogether. It is curious, however, that the same modified form of radula should appear in species of Ovula (e.g. ovum) and that the same absence of radula should occur in species of Eulima (e.g. polita) known to be not parasitic. This fact perhaps points back to a time when the ancestral forms of each group are parasitic and whose radulae were modified or wanting, the modification or absence of that organ being continued in some of their non-parasitical descendants.
Commensalism
Mollusca are concerned in several interesting cases of commensalism, or the habitual association of two organisms, as distinguished from parasitism, where one form preys more or less upon the other.