6o MOLLUSCA AS FOOD FOR FISH AND FOR ONE ANOTHER ch. 



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). 



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 At- 

 lantic, 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 ap- 

 pears 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 some- 

 times 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. Natlca and Nassa pierce in a similar way the shells of 

 Mactra^ Tellina^ Donax^ and Venus. Murex fortispina is fur- 

 nished with a powerful tooth at the lower part of its outer lip. 

 At Noumea, in New Caledonia, its favourite food is Area pilosa^ 

 which lives half buried in coral refuse. The Murex has been 

 seen to drag the Area 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.^ 



The carnivorous land Mollusca, with the exception of Testa- 

 eella, appear to feed by preference upon other snails (pp. 54, 55). 

 1 Fran9ois, Arch. Zool. Exp. Gen. (2) ix. p. 240. 



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. 



