700 



ZOOLOGY 



SECT. 



when the products are mature. Each is an unpaired sac marked 

 by a series of slight lateral .constrictions. There are two gonoducts, 

 each opening immediately in front of the corresponding nephridial 

 duct. 



Little is known of the development of the Aplacophora. The 

 eggs undergo complete segmentation, and give rise to a gastrula 

 by invagination. This develops into a form of trochophore with 

 a ciliated ring, the prototroch. The larva is provided for a time 



with a row of seven 

 calcareous plates on 

 the dorsal surface. 



The eggs of Chiton 

 are fertilised in the 

 mantle-cavity, where 

 in one species they 

 are retained until the 

 embryos are fully de- 

 veloped. In other 

 cases they are laid 

 singly or in strings. 

 The segmentation is 

 complete, and corre- 

 sponds very closely 

 with that of the Poly- 

 chaeta (p. 440). The 

 four megameres give 

 off three quartettes 

 of micromeres, which 

 give rise to the ecto- 



-(pen.GLp 

 .peri.cip 

 ne ph. a. p 



FIG. 616. Chiton, nephridial and genital systems, an- 

 anus : cten. ctenidia ; gen. ap. genital aperture ; gon. gonad ; 



\ ap. 

 peri- 



derm. The mega- 

 meres give origin to 

 the endoderm, and 

 the mesoderm origin- 

 ates in a cell given 

 off by the posterior 

 megamere Mi- 

 crome res and 

 megameres at first 

 arrange themselves in such a way as to form a somewhat 

 flattened blastula, one side of which (vegetal pole) is composed 

 of a comparatively small number of large endoderm cells. Then 

 follows the invagination of the cells of the vegetal side and the 

 resulting formation of a gastrula : this soon becomes elongated 

 in the direction of the future long axis. 



Two rings of cells surrounding the embryo develop cilia (Fig. 

 617, cil.), and owing to the double circlet thus formed an anterior 

 and a posterior region are distinguishable in the larva. The 



