This enormous excess of egg-production in order to ensure the survival of a single pair to replace their parents is a very frequent thing in aquatic animals. But there are many devices by which the necessity for such lavish scattering of a new brood is avoided. The common oyster is already a step in advance of the American in this matter, since it protects its young in the very earliest stages within the shelter of its shell. A further advance in this direction is found in the fresh-water mussels (not to be confused with the very different sea-mussels, since they are bitter and tough, and quite inedible, though used as bait in sea-fishing). The pond-mussel (_Anodon_) and the river-mussel (_Unio_) are of distinct sexes, and the gills of the female become swollen up at the breeding season so as to form two large bags, into which the eggs are laid by her, as many as 500,000 in number. They are fertilised by the sperm filaments discharged by the males, which are carried into the female’s shell by currents produced by the vibrating hairs on the gills, as in the common oyster. But the young remain much longer in the mussel’s gills than do the young oysters in those of their parent’s. Late in the season you find the bag-like gills of the female pond- and river-mussels full of extraordinary little creatures one-thirtieth of an inch long, each provided with a pair of triangular shells. They are discharged into the water, and swim very actively by rapidly opening and shutting the little shells (Fig. 35). The common scallop (_Pecten_, or Pilgrim’s shell) swims every now and then in the same way as do these young mussels, and so do some other bivalves. The young fresh-water mussels produce a long, sticky thread, which trails from the shell (Fig. 35 _by_). Very few have the good chance to get further on in life than this stage, for all depends on their stumbling across fish—a stickleback, or a perch, or a pike—as they blindly snap their shells and wobble through the water. The lucky triangular mite whose sticky thread happens to touch a fish’s body becomes immediately fastened by it to the fish and then grips the skin with its snapping shells, the edges of which are provided with a few long, sharp teeth. The fish probably is quite unaware of the lodgment of the young mussel on its skin, but there it remains, and gets buried for a time in the soft tissues of the fish, becoming thus actually a parasite for some two or three months during the winter season. It nourishes itself on the juices of the fish, and grows to the size of a pin’s head, whilst it is carried away from its birthplace by the peregrinations of its host, the fish. Its shell now ceases to be triangular, and becomes like that of its parents. Eventually the young mussel drops off the fish and rests on the muddy bottom of pond or river, where it remains for many years, growing vastly in size, and barely moving during its long life from the spot where it fell.