FRESH-WATER BIVALVES. 327 



All the species of this family of bivalves breed 

 readily in confinement, during the spring and summer 

 months. They are probably ovo viviparous ; and the 

 young appear to remain for a certain period within 

 the folds of the branchiae previous to their exclu- 

 sion, since many may be found of different sizes 

 within the parent at one and the same time. They 

 have the faculty of producing long before they are 



tion. This phenomenon offers two distinct peculiarities, both 

 which stand in need of explanation. First, how can an animal, 

 specifically heavier than the liquid in which it is immersed, 

 remain thus suspended without falling to the bottom ? In the 

 second place, where does it find a foundation of sufficient support 

 to enable it to move? is it in the liquid itself? or is it against the 

 stratum of air, as some naturalists seem to allow ? This last 

 hypothesis seems to me scarcely tenable, looking to the difference 

 of the density of air and water ; from which it would follow that 

 the resistance to be overcome in order to advance, would be in- 

 comparably greater than the effort produced by resting itself 

 against the air. Moreover, the body of the animal possesses no 

 appendage proper for producing this effort. The little rounded 

 inequalities, or undulations, which the foot forms in contracting 

 itself, cannot serve for this purpose. The same reason appears to 

 me to be an objection to that view which regards the foot as the 

 immediate agent in locomotion, by taking its support or resting- 

 place in the ambient water. In that case, though the undulations 

 of the foot might have a certain hold upon this liquid, the move- 

 ment they produce is very far from active enough to create an im- 

 pulse capable of overcoming the resistance presented by the water 

 to the bulk of the body. 



"But if the entire body were covered with vibratory cilia, 

 there would be no longer anything surprising in seeing the animal 

 use a means of locomotion which serves for the large Plunaria:, 

 and the Nemertes. This fact enables us also to clear up the first 

 of the difficulties which we raised. Undoubtedly the pulmoni- 



