36.4 



THE FRESH-WATER MUSSEL 



the manner of parasites, 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 {sh). The body at the same time becomes cleft from 





/ \ 



Fig. 97. — A, advanced embryo of Anodonta enclosed in the egg-mem- 

 brane. B, free larva or glochidium. 



/, byssus ; g, lateral pits ; j, shell ; sh, hooks ; sm, adductor muscle ; 

 so, sensory hairs ; w, ciliated area. 



(From Korschelt and Heider. ) 



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

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

 is formed a glandular pouch, which secretes a bunch of 

 silky threads, the byssus (/). The larva is now called a 

 glochidium. 



The glochidia, entangled together by means of their 

 byssal threads, escape from the gills of the parent by the 



