ma (Simmia) lives on the latticed blades of seafans, while the root-like bases of the same seafans may be honeycombed with pockets of the Coralliophila shells. Some species of Trivia cowries not only live with the compound ascidians 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 species of Stylifer live embedded in the flesh of starfish, and only a wart-like
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Figure 4. Three stages of parasitism. a, the Pyram snails, Brachystomia, make daily visits to tap the body fluids of the mussel, Mytilus; 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, operculum and mouth parts.
swelling and a bit of shell spire projecting above the surface reveal their presence. One species of Eulima lives inside the intestinal tract of the sea cucumber 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 aperture 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 formation 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
