XII 



PHYLUM MOLLUSCA 



731 



an endoderm cell, gives off externally its daughter-cell of the 

 fourth quartette destined, as in the Polychseta (p. 441), to 

 give rise to the mesoderm. A segmentation-cavity is developed 

 between the micromeres and the megameres, and the result is 

 the formation of the blastula, one side of which (vegetal pole) is 

 greatly thickened owing to its consisting of the large megameres, 

 the opposite side (animal pole) being made up of micromeres. 

 This may become a gastrula by epiboly or over-growth of the 



TTtlC 



7n.es 



FIQ. 653. Diagram of the segmentation and formation of the germinal layers of the Gastro- 

 poda. A and B, lateral view ; C F, viewed from the animal (upper) pole ; H, from the 

 vegetal (lower) pole ; G, in optical section ; eat. ectoderm ; end . endoderm ; mic. micromeres ; 

 meg. megameres ; mes. mesoderm ; poL polar bodies. (After Korschelt and Heider.) 



ectoderm over the megameres ; or, if the segmentation-cavity is 

 of considerable size (Paludina), an invagination takes place. 



The two larval stages, the trochophore and the veliger, are 

 characteristic of the development of the Gastropoda. The former 

 is most typically developed in Patella ; in other Gastropods it 

 undergoes more or less modification. In Patella (Fig. 654) there 

 is a ciliated blastula (A) which has on one side the large megameres. 

 The latter become enclosed by the micromeres, and the foundation 

 of the mesoderm is laid in the manner already described. The 



