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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



tades and eyes disappear; the foot grows out between 

 the mouth and anus, and the gills are developed as a row 

 of ciliated tentacles on each side of the body. With the 

 loss of the velum, the young animal usually settles to the 

 bottom, although there are certain forms which are able to 

 swim throughout life. 



IV. The lawce of Anodonta. The eggs of Anodonta im- 

 plicata pass from the ovary into the gills during the latter 

 weeks of October, and they develop very rapidly. The 

 early stages are -much like those of the oyster, as far as 

 Fig. 169, except that the shell is not at first divided into 

 two valves, but is continuous across the middle line. 

 After this stage is reached, there is little resemblance 

 between the young Anodonta and a marine larva. The 

 shell and mantle develop very rapidly, while the digestive 

 organs become rudimentary, and are not developed until 



five or six months later; in 

 Anodonta implicata, not. until 

 D o the next summer. 



FIG. 174. Anterior view of " Glo- 

 ehidium" larva of Anodonta, enclosed 

 in the egg-shell; magnified about one 

 hundred diameters. (Drawn from na- 

 ture by W. K. Brooks. ) 



b. Byssus. by. Byssus organ, e. Egg- 

 shell. A. Hooks. Is. Left valve of shell. 

 m. Posterior adductor muscle. /*. Hi^ht. 

 valve of shell, s. Setae, e. Velum. 



If a female Anodonta be examined at any time between 

 November 1st and April 1st the outer gills will be found 

 distended by a brownish-red mass, which microscopic ex- 

 amination shows to be made up of the embryos, still en- 

 closed in the egg-shells. One of them is shown from in 

 front in Fig. 174, and in ventral view, after the removal 



