XII 



PHYLUM MOLLUSCA 



747 



latter, consisting of the greater part of the yolk enclosed in a thin 

 layer of blastoderm, forms a rounded appendage of the embryo the 

 yolk-sac (yk. s.). The yolk-sac undergoes contractions, which are 

 due to the action of contractile cells in the thin mesoderm 

 lining it, r and by this means the yolk is forced into the interior of 

 the body^of the embryo. 



iFio. 659. Early stages in the development of IiOligO. A, stage at which the rudiments of the eyes 

 and of the shell-gland are first distinguishable ; B, later embryo from the oral side ; C and D, 

 from the anal side. ant. f. f. anterior funnel fold ; ar. rudiments of arms ; cten. ctenidia ; 

 eye, eye ; mo. mouth ; mant. rudiment of mantle ; ot. otocyst ; post. f. f. posterior funnel fold ; 

 $li. -jl. shell-gland ; yk. s. yolk-sac. (After Korschelt and Heider.) 



The anus appears as an aperture situated on a little papilla the 

 anal papilla. A row of cilia, which are developed in the neighbour- 

 hood of the mouth in some forms, perhaps represent the velum 

 or prse-oral circlet of other molluscan embryos. The mantle now 

 increases in extent, and its margins become more prominent. 

 'The anterior funnel folds grow out, and become united in the 



