146 DEVELOPMENT OF LARVAL PELECYPODA chap. 



Development of Fresh- water Bivalves. — The vast majority 

 of fresh-water bivalves either pass tlie larval stage entirely 

 within the mother, and do not quit her except in a perfectly 

 developed form (^Cydas^ Pisidium)^ or assume a mode of de- 

 velopment in which free larvae indeed occur, but are specially 

 modified for adaptation to special circumstances ( Unio'). Qyclas 

 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 extraodinarily 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 Lov^n.i 



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. 



Larvae of Unionidae. — The early stages of the develop- 

 ment of Unio and Anodonta (so far as the species of North 

 America, Europe, and Asia are concerned) is of extreme interest, 

 from the remarkable fact that the young live for some time 

 1 Kon. Vet. Akad. Handl. 1848, pp. 329-4^5. 



