146 DEVELOPMENT OF LARVAL PELECYPODA chap. 



of fresh -water bivalves either pass the larval stage entirely 

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

 developed form {Cydas, Pisidium), or assume a mode of develop- 

 ment in which free larvae indeed occm% but are specially modified 

 for adaptation to special circumstances {Unio). Cydas and 

 Fisidnwi, 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 Cydas. 



The larva of JDreissensia (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 Em'ope (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, 

 wdiich 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 Carduim, as figured 

 by Loven.^ 



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 captm-ed 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 Cydas. 



Larvae of Unionidae. — The early stages of the development 

 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 para- 

 sitically attached to certain species of fresh-water fishes. In 



1 Kon. Vet. Akad. Handl. 1848, pp. 329-435. 



