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 Algz (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 Mans, 
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 
