410 



THE MUSSEL 



CHAP. 



of the outer gill-laminae, which they distend enormously. 

 Thus the outer gill-laminae act as brood-pouches, and in 

 them the embryo develops into the peculiar larval form 

 presently to be described. 



As in the frog and earthworm, the cells formed by the 

 segmentation of the oosperm are of two sizes, small 

 cells composed entirely of protoplasm, and large cells 

 loaded with yolk-granules. The large become enclosed 

 by the small cells, but the enteron thus formed is very 

 small and quite unimportant during early larval life, the 



A. 



B. 



.t. 



FIG. 105. A, advanced embryo of Anodonta enclosed in the egg-membrane. B, free 



larva, or glochidium. 



/ byssus s. shell ; sh. hooks ; sm. adductor muscle ; so. sensory hairs ; w. 

 cilia ted 'area. (From Korschelt and Heider's Embryology.) 



young mussels being nourished, after the manner of 

 parasites (p. 280), by a secretion from the gills of the 

 parent. 



The dorsal surface of the embryo is soon marked out 

 by the appearance of a deep depression, the shell-gland, 

 which secretes, in the first place, a single, median shell. 

 This is, however, soon replaced by a bivalved larval 

 shell (Fig. 105, s) of triangular form, the ventral angles 

 being produced into hooks (sh). The body at the same 

 time becomes cleft from below upwards (A), forming 

 the right and left mantle-lobes. On the ventral surface, 



