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 trochosphere stage with a pra3-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, whilen 

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

 As yet there is no anus, that aperture, with the proct 

 formed by invagination at a later stage. The apical 

 present in the early larva; but the rudiments of 



B 



cil 



mese,nt 



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

 ti-ochosphere. calc. calcifications (rudiments of shell) ; cer. g. cerebral ganglion ; cil. ciliary ring ; 

 til. t. ciliary tuft at apical pole ; eye. eye ; ft. gl. foot-gland ; mes. mesoderm ; mesent. mesenteron ; 

 mo. mouth ; rd. radular sac ; sp. spines ; vis. g. visceral ganglion. (From Korschelt and Heider, 

 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 ccelomic space between them. The post-oral part 

 of the embryo now presents an appearance resembling rudimentary 

 segmentation. This is due 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 



