In East Africa a species of Ichneumon (Herpestes fasciatus) devours snails, lifting them up in its forepaws and dashing them down upon some hard substance.[139] In certain islands off the south coasts of Burmah, flat rocks covered with oysters are laid bare at low tide. A species of Monkey (Macacus cynomolgus) has been noticed to furnish himself with a stone, and knock the oysters open, always breaking the hinge-end first, and then pulling out the mollusc with his fingers.[140]
The walrus is said to support himself almost entirely on two species of Mya (truncata and arenaria), digging them out of the sand, in which they live buried at a depth of about 1½ feet, with his powerful tusks. Whales swallow enormous numbers of pelagic molluscs (Clio, Limacina), which are at times so abundant in the Arctic seas, as to colour the surface for miles. Many of the larger Cetacea subsist in great part on Cephalopoda; as many as 18 lbs. of beaks of Teuthidae have been taken from the stomach of a single Hyperoodon.
Fish are remarkably partial to Mollusca of various kinds. The cat-fish (Chimaera) devours Pectunculus and Cyprina, crushing the stout shells with its powerful jaws, while flounders and soles content themselves with the smaller Tellina and Syndosmya which they swallow whole. As many as from 30 to 40 specimens of Buccinum undatum have been taken from the stomach of a single cod, and the same ‘habitat’ has been recorded for some of the rarer whelks, e.g. Bucc. humphreysianum, Fusus fenestratus, the latter also occurring as the food of the haddock and the red gurnard. No less than 35,000 Turtonia minuta have been found in the stomach of a single mullet. Nudibranchs are no doubt dainty morsels for fish, and hence have developed, in many cases, special faculties for concealment, or, if distasteful, special means of remaining conspicuous (see pp. [71–74]).
Fig. 22.—Two valves of Mytilus edulis L., representing diagrammatically the approximate position of the holes bored by Purpura in about 100 specimens of Mytilus, gathered at Newquay, Cornwall.
Besides the dangers to which they are exposed from other enemies, many of the weaker forms of Mollusca fall a prey to their own brethren. Nassa and Murex on this side of the Atlantic, and Urosalpinx on the other, are the determined foes of the oyster. Purpura lapillus prefers Mytilus edulis to any other food, piercing the shell in about two days’ time by its powerful radula, which it appears to employ somewhat in gimlet fashion. If Mytilus cannot be procured, it will eat Littorina or Trochus, but its attempts on the hard shell of Patella are generally failures. The statement which is sometimes made, that the Purpura makes its hole over the vital parts of the Mytilus, appears, according to the evidence embodied in the annexed figure, to be without foundation. The fact is that a hole in any part of its shell is fatal to the Mytilus, since the long proboscis of the Purpura, having once made an entrance, can reach from one end of the shell to the other. The branchiae are first attacked, the adductor muscles and edges of the mantle last. Natica and Nassa pierce in a similar way the shells of Mactra, Tellina, Donax, and Venus. Murex fortispina is furnished with a powerful tooth at the lower part of its outer lip. At Nouméa, in New Caledonia, its favourite food is Arca pilosa, which lives half buried in coral refuse. The Murex has been seen to drag the Arca from its place of concealment, and insert the tooth between the valves, so as to prevent their closing, upon which it was enabled to devour its prey at leisure.[141]
The carnivorous land Mollusca, with the exception of Testacella, appear to feed by preference upon other snails (pp. [54], [55]).
Parasitic Worms, Mites, etc.—A considerable number of the Trematode worms pass one or more of the stages in the cycle of their development within the bodies of Mollusca, attaining to the more perfect or sexual form on reaching the interior of some vertebrate. Thus Distoma endolabum Duj. finds its first intermediate host in Limnaea stagnalis and L. ovata, its second in L. stagnalis, or in one of the fresh-water shrimps (Gammarus pulex), or in the larvae of one of the Phryganeidae (Limnophilus rhombicus), attaining to the sexual form in the common frog. Distoma ascidia v. Ben. passes firstly through Limnaea stagnalis or Planorbis corneus, secondly through certain flies and gnats (Ephemera, Perla, Chironomus), and finally arrives within certain species of bats. Distoma nodulosum Zed. inhabits firstly Paludina impura, secondly certain fishes (Cyprinus Acerina), and lastly the common perch. The sporocyst of Distoma macrostomum inhabits Succinea putris, pushing itself up into the tentacles, which become unnaturally distended (Fig. [23]). While in this situation it is swallowed by various birds, such as the thrush, wagtail, and blackbird, which are partial to Succinea, and thus obtains lodgment in their bodies. Amphistoma subclavatum spends an early stage in Planorbis contortus, after which it becomes encysted on the skin of a frog. When the frog sheds its skin, it swallows it, and with it the Amphistoma, which thus becomes established in the frog’s stomach.[142]
Fig. 23.—A Trematode worm (Leucochloridium paradoxum Car.) parasitic in the tentacles of Succinea putris L. × 20 (after Baudon).