XII 



PHYLUM MOLLUSCA 



643 



mes 



.mes 



Development. Segmentation of the oosperm is complete, but 

 unequal. A gastrula is formed by the invagination of the mega- 

 meres into the micro- 

 meres, but the archen- ;rk 

 teron (Fig. 533, ent.) 

 thus formed is quite 

 small and insignifi- 

 cant, and has no 

 physiological import- 

 ance until a late period 

 of larval life. Certain 

 of the cells of the 

 gastrula are budded 

 off into the blasto- 

 ccele, where they ac- 

 cumulate and form 

 the mesoderm (mes.). 

 At about the same 

 time a deep invagina- 

 tion (sd.) is formed, 

 which might easily be 

 mistaken for the ar- 

 chenteron, but is really a very characteristic molluscan organ, 

 the shell-gland : it marks the dorsal surface of the embryo. The 

 posterior end is distinguished by a tuft of long cilia. 



The shell-gland becomes converted into a plate of long, cylin- 



R. B. 



jnes 



FIG. 533. Early embryo of Anodonta. e/i, vitelline 

 membrane; ent. archenteron ; m. micropyle ; mes. meso- 

 derm ; rk, polar cells ; sd, shell-gland ; sk, lateral cells ; 

 ic, cilia, (From Korschelt and Heider's Embryology.) 



mes. 



534. Two later stages in the development of Anodonta. e;it. archenteron ; me*, meso- 

 derm ; s, shell ; sd, shell-gland ; so. sense-organs ; v:, cilia. (From Korschelt and Heider's 

 ology.) 



drical cells (Fig. 534, sd:), from which an unpaired shell (s.) is 

 secreted. This is 'replaced before long by a bivalved shell of 

 triangular form, its ventral angles produced? into incurved hooks 



m m^ 





T T 



