=Development of Fresh-water Bivalves.=--The vast majority of fresh-water bivalves either pass the larval stage entirely within the mother, and do not quit her except in a perfectly developed form (_Cyclas_, _Pisidium_), or assume a mode of development in which free larvae indeed occur, but are specially modified for adaptation to special circumstances (_Unio_). _Cyclas_ and _Pisidium_, and no doubt all the kindred genera, preserve their ova in a sort of brood-pouch within the gills, in which the ova pass the earlier stages of their development. But, even so, the larva of these genera retains some traces of its original free-swimming habits, for a rudimentary velum, which is quite useless for its present form of development, has been detected in _Cyclas_. The larva of _Dreissensia_ (see Fig. 47, A), so far as is at present known, stands alone among fresh-water bivalves in being free-swimming, and to this property has been attributed, no doubt with perfect justice, the fact of the extraordinarily rapid spread of _Dreissensia_ over the continent of Europe (chap. xvi.). In expelling the ova, the parent slightly opens the shells and then quickly closes them, shooting out a small point of white slime, which is in fact a little ball of eggs. The general course of development is precisely parallel to that of marine _Pelecypoda_, greatly resembling, so far as form is concerned, certain stages in the growth of the larvae of _Modiolaria_ and _Cardium_, as figured by Lovén.[263] In June and July the larvae appear in large numbers on the surface of the water, when in spite of their exceedingly small size, they can be captured with a fine hand-net. They pass about eight days on the surface, feeding apparently on minute floating algae. During this time, the principal change they undergo is in the formation of the foot, which first appears as a small prominence midway between the mouth and anus, and gradually increases in length and flexibility. When the larva sinks to the bottom, the velum soon disappears entirely, the foot becomes exceedingly long and narrow, while the shell is circular, strongly resembling a very young _Cyclas_.