Enormous as is the output of young by a single oyster—amounting to something like a million a year in probably four or five successive years—yet it must be remembered that on the whole, taking all the various oyster-beds into account, some of which increase whilst others dwindle or actually die out altogether, there is no increase in the oyster population of the seas. Taking them all round, five million young oysters start life in order that one may finally come to maturity, so many and varied and incessant are the dangers, the predatory enemies, the destructive effects of cold currents, bad ground, and other chances of life and death on to which the swarming swimming young are launched. The above brief history applies to the North Sea or Channel oyster, which is also found (with other species) in the Mediterranean. The American and the Portuguese oyster differ from it in being of distinct sexes, and in the fact that the eggs are discharged into the sea by the females, and are there fertilised by the sperm discharged by the male oysters, instead of in the parent’s body. Other “molluscs,” such as snails and whelks, enclose their fertilised eggs, when they lay them, in egg-shells. Some snails enclose a single egg in a shell which is filled up with clear liquid—corresponding to the “white” of a bird’s egg—in which the egg floats and develops. The eggs of the common snail are not bigger than a hemp-seed, but some Indian snails lay eggs as big as those of a robin, with a hard, calcareous shell, and the young snail has quite a large coiled shell of its own before it escapes from the egg-shell. So that it looks, when one of the big snail’s eggs is broken, as though a snail had managed to get inside a bird’s egg without making a hole in it! The whelks and their kind lay many eggs in one shell or capsule, and the sea-slugs produce a sort of firm jelly, in cords like vermicelli, the jelly enclosing hundreds of little sacs filled with liquid, in which the true germs or fertilised egg-cells float. These are all methods for protecting the young in their earliest condition. One of our pond-snails—the _Paludina_—keeps her eggs, whilst they develop, inside the dilated end of the tube which leads from the egg-producing organ or ovary to the exterior. The young snails nearest the opening to the exterior are the furthest advanced in development, and are as big as a dried pea. All stages, from the minute germ just fertilised to well-formed young, may be found in these snails, and the whole course of their development and gradual change and growth can be minutely studied with the microscope in one specimen.