314 MARINE INVERTEBRATES 
there find a comfortable shelter for life. A numerous class of 
pelagic or free-swimming mollusks supposedly never go to the 
bottom at all, but spend the whole of their existence on or near 
the surface of the sea, always in open water, where their fragile 
shells may not be injured by rough contact with solid substances. 
The food of some is vegetable, of others, animal. The bivalves, 
like the clams, oysters, cockles, and mussels, feed only on micro- 
scopic organisms. They create a current of water through their 
siphons, or mantle openings, and then, by a process best known to 
themselves, catch and swallow all the animalcule thus brought 
to their mouths. The univalves which possess a siphon are, for 
the most part, carnivorous, and are often most voracious crea- 
tures. They feed upon any animal matter they can find, while some. 
of them are enabled by means of a sharply toothed tongue to bore 
through the solid shells of other mollusks and extract the suceu- 
lent vitals from within. One energetic little mollusk in particular, 
Urosalpins cinerea, is for this reason a great pest upon the oyster- 
beds. Univalves not possessed of a siphon may generally be 
considered herbivorous; they pass most of their time peacefully 
browsing upon alge. 
Mollusks are all oviparous or ovoviviparous; that is, they lay 
eggs, or, laying eggs, they retain them within their shells until the 
young are hatched out. As a rule, each species of mollusk has 
its own particular method of protecting its eggs from external 
injury. Some construct tough, leathery capsules which are strung 
together in various patterns. The egg-capsules of Purpura lapil- 
lus, resembling little pinkish or yellowish club-shaped stalks, may 
frequently be found in the crevices of rocks and under the rock- 
weeds. The egg-cases of Polynices (Lunatia) are most peculiar, 
resembling inverted gelatine-bowls with the bottom knocked out; 
when wet they are semi-elastic translucent masses in which 
may be seen myriads of eggs. Buccinum undatum arranges its 
hemispherical egg-capsules in layers one above the other. The 
number of eggs so deposited is often very great, running well into 
the thousands. Egg-capsules of Fulgw are leathery coils of 
angular disks adhering by one edge to a connecting-band of a 
similar texture. (See Plate I.) 
