122 THE LIFE OF THE MOLLUSCA 



resemble a small mass of that substance. The 

 shells of the true Water Snails are often quite over- 

 grown with minute green weed, and so, too, are 

 those of marine Gastropods. The latter, moreover, 

 are frequently encrusted with calcareous Algse (Nuli- 

 pores, etc.), or overgrown with some Hydractinian 

 till they are unrecognizable. A more curious and 

 striking instance is offered by the genus Xenophora, 

 the members of which build on to the upper surface 

 of the shell, especially at its margin, fragments of 

 stone, coral, or other shells — whatever, in fact, is 

 common on the sea-floor where they happen to be, 

 after the well-known manner of the Caddis-worm 

 (Plate XXVIII., Fig. 3). 



Among the Bivalves some species of Lima and 

 Volsella (= Modiolus) attach various bits of marine 

 refuse to themselves by threads of the byssus till 

 they are more or less concealed thereby. Lima hians, 

 indeed, is credited with a regular " nest " of this sort. 



Of other forms of protection of a more active 

 nature may be mentioned the possible advantage of 

 spiny shells in preventing the occupant from being 

 swallowed for food : the poisonous bite of the Cones 

 may likewise serve in good stead. 



It is among the more defenceless Opisthobranchs, 

 however, that the best instances have so far been met 

 with. 



Oscanius is known to secrete an acid from the 

 surface of the body. Ancula, Doto, and many of the 



