Life of the Snails 19 



ma {Smtnia) lives on the latticed blades of seafans, while the root-like bases 

 of the same seafans may be honeycombed with pockets of the Coralliopbila 

 shells. Some species of Trivia cowries not only live with the compound as- 

 cidians or sea squirts (Botryllus) but also feed upon them. Deep holes are 

 eaten into the ascidian in which the female snail deposits her flask-shaped egg 

 capsules (fig. 9). Among the Eulima and tiny Pyram snails there are many 

 species which parasitize sea urchins and certain kinds of clams. Several spe- 

 cies of Sty lifer live embedded in the flesh of starfish, and only a wart-like 



Figure 4. Three stages of parasitism, a, the Pyram Snails, Brachystoviia, make 

 daily visits to tap the body fluids of the mussel, Mytihis; b, the adult of the 

 Stylifer Snail becomes encased in the tissues of the starfish; c, the Entocolax Snail 

 is embedded in the flesh of a holothurian sea-cucumber and has lost shell, oper- 

 culum and mouth parts. 



swelling and a bit of shell spire projecting above the surface reveal their pres- 

 ence. One species of Eulima lives inside the intestinal tract of the sea cucum- 

 ber and obtains its food by tapping the nutritious juices of its host by means 

 of a modified, syringe-like snout. 



HOW THEY GROW 



In most cases the shell material in the snails is secreted by special glands 

 located along the edge of the fleshy mantle of the animal. Within the aper- 

 ture or mouth of the shell a certain amount of reinforcing material may be 

 secreted by the roof of the mantle, especially in the case of the heavy trochid 

 shells which are nacreous within the aperture. The foot is often the source 

 of shell material, not only as the site of the formation of the hard trapdoors 

 or opercula of the turban and natica shells but also as an important addition 

 to the shell itself. The actual formation of calcium carbonate and the forma- 

 tion of the various layers are discussed in more detail in the chapter on clams. 



In some groups of gastropods, particularly certain wentletraps and liotias, 

 the mantle edge is capable of producing exquisitely fine filigree or porous 

 shell structure whose intricate designs and overlapping layers can best be 

 seen with the aid of a magnifying glass. In the cowrie shells, the mantle has 

 two large extensions which are spread at will over the entire outside of the 



