252 



COMPARATIVE ANATOMY 



circular calcareous spicules, covering it laterally ; the ventral side is, however, 

 naked. A mouth is still wanting, the endodermal mass is not yet hollow, and on 

 each side, between the endoderm and the integument, there is a solid mesodermal 

 streak. 



B. Gastropoda. 



As a type of the development of the Gastropoda, we may take Paludina vivipara 

 (Figs. 209 and 210), the ontogeny of which has recently been again very carefully 

 investigated. Development here takes place within the body of the mother. The 

 egg is comparatively poor in yolk. A coelogastrula is formed by invagination, the 

 blastopore of which marks the posterior end of the germ, and becomes the anus. 

 No proctodeum is formed. The whole of the intestine from the stomach to the 



Fig. 20!!.— Development of Paludina vivipara (alter v. Erlanger). A and B, Stage after 

 gastrulation, with tlie rudiments of tlie mesoderm and the ccelom as outgrowths of the archenteron. 

 A, Median optical longitudinal section. B, Horizontal optical longitudinal section. C, Horizontal 

 optical longitudinal section through the embryo, after the entire separation of the coelomic sac 

 from the intestine. D, Sagittal optical longitudinal section through an embryo, in which the 

 mesoderm lias broken up, the cells becoming spindle-shaped. 1, Velum ; 2, segmentation cavity ; 

 3, archenteron ; 4, coelom ; 5, blastopore ; 6, mesoderm cells ; 7, shell-gland. 



anus proceeds from the endoderm. The mesoderm arises as a ventral hollow out- 

 growth of the archenteron, which soon becomes constricted from the intestine, and 

 lies between the intestine and the ectoderm in the segmentation cavity as a vesicle 

 with two points directed forward (Fig. 209 A, B, C). This vesicle spreads out to 

 the right and left dorsally round the intestine, finally closing round it dorsally. Its 

 outer wall of cells, which becomes applied to the ectoderm, forms the parietal 



