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Rh eggs may develop in an undisturbed, food-laden medium. Very frequently extra eggs (nurse eggs) are added which serve as food for the young that hatch first. The young may emerge from the egg cases as miniature replicas of their parents and commence a life of crawling and feeding, or they may escape as free-swimming larval forms. The latter are known as veligers and possess special organs for swimming. The larval shell is often quite different from the adult shell and, in some species, there may be an extra shell or echinospira encasing the entire veliger.

There are many types of egg cases, and some of these are illustrated in figures 8 and 9; others are briefly described under the generic or family discussions in the identification section. Several types of egg-laying may be found within a single family or even genus.

In some groups of snails which are more or less sedentary, the egg capsules may be protected by the female. In the cap-shell, Hipponix, the underside of the foot of the female has a tough, reinforced ridge of flesh to which she attaches her gelatinous egg sacs. In some worm-shells, Vermetus, whose shells are permanently attached to the rocks, the eggs are deposited on the inside of the female’s own shell.

The time and length of breeding differs among mollusks depending mainly on the geographical locality, the temperature of the water, phases of the moon and the inherent characteristics of the species. Some species spawn once a year for a few weeks only, while others may produce eggs half of the year as long as the temperature is suitable.

The eggs, larvae and young have been described for many species by