The food of some is vegetable, of others, animal. The bivalves, like the clams, oysters, cockles, and mussels, feed only on microscopic 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 animalculæ thus brought to their mouths. The univalves which possess a siphon are, for the most part, carnivorous, and are often most voracious creatures. 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 succulent 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 algæ. 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 lapillus/, resembling little pinkish or yellowish club-shaped stalks, may frequently be found in the crevices of rocks and under the rockweeds. 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 /Fulgur/ are leathery coils of angular disks adhering by one edge to a connecting-band of a similar texture. (See Plate I.) [pg315] In bivalves the eggs are retained within the parent shell until hatched, and the young, which then escape into the water, are very different-looking creatures from the parent. Generally the young bivalve is free-swimming and exceedingly lively in its movements; sometimes by means of cilia it is enabled to attach itself to any passing host, and thus the species is widely disseminated. The fry soon lose their embryonic form and take on the characteristics of the adult. The longevity of mollusks varies greatly with the species. Some attain an age of from ten to fifteen years, while some are only annuals. Oysters continue to grow for four or five years, after which they may survive many years more. In truth, little is known concerning the duration of life even of our better-known shore species.