LAMELLIBRANCHIATA. 393 



coalescence of two papillae. It is never produced into lobes, as in 

 Gastropoda and Pteropoda. In some fresh-water forms which have 

 no free larva the velum is reduced (Anodonta, Unio, Cyclas\ or aborted 

 (Pisidium). In these forms, in Teredo and Ostraea edulis, the central 

 flagellum is absent. 



b. Archenteron inclosed by hypoblast cells. 



c. Body cavity (?). 



d d. Epiblast. The cells forming this layer divide rapidly and surround 

 the large, slowly dividing hypoblast cells, thus forming a Gastrula by 

 overgrowth or epibole. 



In an early stage the embryo is covered with short cilia. In a later stage than 

 the one figured the two shell-valves appear, at first meeting without a hinge. They 

 cover the velum which becomes retractile. The mantle-folds grow. The archen- 

 teron acquires hepatic diverticula ; a stomodaeum and proctodaeum are formed ; 

 and the foot grows out between mouth and anus. The anterior appears before 

 the posterior adductor. At this stage the embryo is hatched. 



Cf. Loven, Archiv fur Naturgeschichte, 15, i, 1849, p. 317; Balfour, Com- 

 parative Embryology, vol. i. 1880, pp. 214, 216-17. 





