XII 



PHYLUM MOLLUSCA 



675 



beset with spines (Fig. 583, sh). At the same time the body of 

 the larva, which has hitherto been an undivided mass projecting 

 between the two valves of the shell, becomes cleft from below 

 upwards, and thus divided into a single dorsallv-placed body 



R. B. 



so. 



mes 



TO--' 



'- enf. 





^o-O. 



FIG. 582. Two later stages in the development of Anodonta. rnt. archenteron ; mes. meso- 

 derru ; s. shell ; sd. shell-gland ; so. sense-organs ; w, cilia. (From Korschelt'and Heider's 

 Embryology.) 



proper, and paired right and left mantle-lobes. Upon the latter 

 peculiar brush-like sense-organs make their appearance, and on 

 the ventral surface of the body is formed a glandular pouch, which 

 secretes a long thread, the provisional byssus (/). The mesoderm 



B. 



J. 



So... 



- ;- , 



5- 



sm 



FIG. 583. A, advanced embryo of Anodonta. B, free glochidium. /, provisional byssus 

 s. shell ; sh. hooks : sm. adductor muscle ; so. sense-organs ; w. cilia. (From Korschelt and 

 Heider's Embryology.) 



cells give rise to a single immense adductor muscle (sm}, the fibres 

 of which extend from valve to valve. 



The larva is now called a glochidium : it remains in the brood- 

 pouch, nourished by a secretion from the walls of the latter, and 

 entangled with its fellows by means of the byssus. At this stage 

 the outer gill-lamina appears as if stuffed full of closely 

 aggregated sand-grains. Before long the larva) are ejected 



