FRESH-WATER ANIMALS 93 



horizontal bars. The two laminae are further 

 bolted together at intervals by cross bars. The 

 eggs, which are exceedingly numerous, form a 

 bulky mass lying between the two laminae. Owing 

 to the cilia which clothe the outer surface of the 

 bars, streams of water are constantly passing 

 through the meshes of the trellis-work ; and in this 

 way the embryos not merely obtain perfect pro- 

 tection during the early stages of development, but 

 are also abundantly supplied both with oxygen for 

 respiration, and with food in the shape of minute 

 particles of vegetable or animal matter suspended 

 in the water. The outgoing stream will also serve 

 to carry away any excretory or faecal matter passed 

 out by the embryos. 



At the time of leaving the parent the young 

 Anodonta is a fully formed bivalve, but is still very 

 unlike the parent, and of too small size, and too 

 weak to hold its own against the currents of the 

 streams in which the adult lives. The young at 

 this period have bivalved shells, the two halves of 

 which are triangular, with the apices incurved so as 

 to form sharp serrated projections, which, when the 

 valves are closed, form a very efficient pair of 

 pincers ; they have a very rudimentary foot and gills, 

 and in other respects differ markedly from the 

 adult. 



On passing out from the gills of the parent into 

 the water, they swim by snapping movements of 

 the valves, like a Pecten, but very speedily attach 

 themselves by the pincer-like processes of their 

 shells to fish, such as the stickleback, or else to the 

 legs of water birds. The skin either of the fish or 



