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 succu- 

 lent vitals from within. One energetic little mollusk in particular, 

 Urosalpinx 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 algas. 



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. Bucdnnm 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 Fulgur are leathery coils of 

 angtilar disks adhering by one edge to a connecting-band of a 

 similar texture. (See Plate I.) 



