DEVELOPMENT 



395 



which they distend enormously. Thus the outer gills 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 young mussels being 



B. 



- sh. 



Fr;. 97- — A, :id\aiiced embryo of .\iiudoiua t;iicIo>cJ in the cgg-iTienibrane. I!, free 

 !ar\'a or glochidium. 

 /*. - byssus ; ^: lateral pits ; s. shell ; s/t. hooks ; s!?/. adductor muscle ; so. sensory 

 hairs ; lu. ciliated area. (From Korschelt and Heider.) 



nourished, after the manner of parasites (p. 271), 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. 97, 

 s') of triangular form, the ventral angles being produced into 

 hooks {sli). The body at the same time becomes cleft from 

 below upwards (a), forming the right and left mantle-lobes. 

 On the ventral surface, between the lobes of the mantle, is 



