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

 the body of the embryo. 



!FiG. 659. Early stages in the development of Loliero . A , stage at which the rudiments of the eyes 

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

 from the anal side. nut. /. /. anterior funnel fold; or. rudiments of arms; c>cn. ctenidia ; 

 < in , eye ; mo. mouth ; tint at. rudiment of mantle ; ot. otocyst ; post. f.f. posterior funnel fold ; 

 tli. <il. shell-gland ; >ik. .<. 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 prae-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 



