1'HYLUM MOLLUSCA 



701 



blastopore becomes shifted from its original posterior position 

 forwards on the ventral surface until it conies to be situated just 

 behind the circlet of cilia ; it undergoes elongation, and an invagina- 

 tion of ectoderm round its anterior end forms the mouth (mo.) and 

 stomodaeum. 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 

 trochophore stage, with a pre-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 proctodaeum, being formed later by invagination. 

 An apical plate is not present in the early larva ; but the rudiments 

 of the cerebral ganglia (C, cer. </.), which subsequently appear at 

 the apical pole, probably represent it. Primitive nephridia, such 



B 



nl 



men 



mesrnf 



FIG. 617. Chiton, development. A, general view of larva ; B, section of early, and C, of 

 later trochophore. calc. calcifications (rudiments of shell) ; cer. g. cerebral ganglion ; oil. 

 ciliary ring; oil. t. ciliary tuft at apical pole ; eye, eye ; /(. gl. foot-gland ; mes. mesoderm ; 

 mesent. mesenteron ; mo. mouth ; rd. radular sac ; up. spines ; vise. g. visceral ganglion. 

 (From Korschelt and Heider. after Kowalewsky.) 



as occur in Annulate and many Molluscan trochophores, are not 

 present. 



The post-oral region now becomes greatly elongated ; the meso- 

 derm increases greatly in extent, and forms two well-defined streaks, 

 which are afterwards 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 seg- 

 mentation. This is due to the development of a series of rudiments 

 of the eight pieces of the shell (A, calc.}, each of which is 

 formed independently after the fashion of the entire shell of other 

 Mollusca. 



Ethology, Distribution, &c. All the Arnphineura are 

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



