670 



ZOOLOGY 



SECT. 



shifted from its original posterior position forwards on the ventral 

 surface, until it comes to be situated just behind the circlet of 

 cilia ; it becomes elongated, and an invagination of ectoderm round 

 its anterior end forms the mouth (mo.} and stomodseum. A ventral 

 diverticulum of this forms the rudiment of the radular sac (rd.). By 

 greater relative growth of the post-oral part the embryo assumes 

 the form of a pear ; and in this trochosphcre stage with a prse-oral 

 circlet and a bunch of cilia in the middle of the apical area, 

 it becomes free in the case of certain of the species, while in others 

 it remains enclosed in the egg up to a later stage of development. 

 As yet there is no anus, that aperture, with the proctodasum, being- 

 formed by invagination at a later stage. The apical plate is not 

 present in the early larva; but the rudiments of the cerebral 



B 



mes 



mes 



mzse.nl 



FIG. 570. Chiton, development. A, general view of larva; B, section of early, C, of later 



after Kowalewsky.) 



ganglia (C,cer.g.}, which appear at the apical pole at a later stage, 

 probably represent it. Primitive nephridia, such as occur in 

 Annulate and many Molluscan trochospheres, are not present. 



The post-oral region now becomes greatly elongated ; the 

 mesoderm increases greatly in extent, and forms two well-defined 

 streaks, which afterwards become divided into parietal and visceral 

 layers with a coelomic space between them. The post-oral part 

 of the embryo now presents an appearance resembling rudimentary 

 segmentation. This is clue to the development of a series of 

 rudiments of the eight pieces of the shell (B, calc.\ each of which 

 becomes formed independently after the fashion of the entire shell 

 of other Mollusca. 



Ethology, Distribution, &c. All the Amphineura are ma- 

 rine. The Placophora occur at all depths, though most abundant 



