XII 



PHYLUM MOLLUSCA 



043 



eh 



Development. Segmentation of the oosperm is complete, but 

 unequal. A gastrula is formed by the imagination of the in> 

 meres into the micro- 

 meres, but the archeii- 

 teron (Fig. 533, cut.) 

 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- 

 coele, where they ac- 

 cumulate and form 

 the mesoderm (mcs.). 

 At about the same 



time a deep invagllia- Fl(: - SSS. Early embryo of Anodonta. ..A, vitelline 



/ 7\ i membrane ; ent. archenteron ; i. micropyle ; <..<. meso- 



tlOn (SO,,) IS lOrmedj derm; rk, polar cells ; scl, shell-gland ; sic, lateral cells ; 



which might easily be "'' cilia ' (From Korachelt and Heider ' s Em 



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- 



mes - 



R. 



B. 



JD. ' 



\ g 



, .y <?&/ 



ad. 



-70. 



FIG. 534. Two later stages in the development of Anodonta. <*,i,t. archentenm : mes. i 



derm; s, shell; sd, shell-gland; so. sense-organs; w, cilia. (From Korscholt :unl llci'lfi -- 

 Embryology.) 



drical cells (Fig. 534, scL), 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 



rp riilfc 



