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). [Illustration: 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).