364 THE FRESH-WATER MUSSEL LESS. 



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 (Pig. 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. 



f, -byssus ; g, lateral pits ; s y shell ; s/i, hooks ; .?;//, 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 byssrts (/). 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 



