6o MOLLUSCA AS FOOD FOR FISH AND FOR ONE ANOTHER 



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 Ijrethren. Nassa and 

 Murex on this side of the At- 

 lantic, and Urosaljnnx on the 

 otlier, are the determined foes 

 of the oyster. Pmjnira hqnllns 

 prefers Mytilus eclulis to any 

 otlier food, piercing the shell in 

 iihout two days time l)y its 

 powerful radula, which it ap- 

 pears to employ somewhat in 

 gimlet fashion. If Mytilvs 

 cannot he procured, it will eat 

 Littorina or Trochvs, l)ut its 

 attempts on the hard shell of 

 The statement which is some- 

 times made, that the' Fuiyura 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 Pur2mra, having once made an entrance, 

 can^'reach from one end of the shell to the other. The liranchiae 

 are fiirst attacked, the adductor muscles and edges of the mantle 

 last. Natica and Nassa pierce in a similar way the shells ot 

 Maetra, Tdlina, Donax, and Vcmis. Murex for tispina is furnished 

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

 Noumea, in New Caledonia, its favourite food is Area jnlosa, 

 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 leisiu^e.^ 



The carnivorous land Mollusca, with the exception of Te^sta- 



cella, 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 



1 Franc;oi.s, Arch. Zool. Exp. Gen. (2) ix. p. 2-10. 



Fig. 22. — 'Two valve« of Mytilus edulis 

 L., representing diiigramniatically the 

 appro.ximate position of the holes 

 bored by Purpura in about 100 speci- 

 mens of Mytilus, gatliered at New- 

 qnay, Cornwall. 



Peitella are generally failures. 



