'20 



ZOOLOGY 



SECT. 



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 proctodseum, 

 being formed later by invagination. An apical plate is not 

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

 ganglia (C, cer. g.), which subsequently appear at the apical pole, 

 probably represent it. Primitive nephridia, such as occur in 

 Annulate and many Molluscan trochophores, are not present. 



The post-oral region now becomes greatly elongated ; the 

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



B 



dL.t 



mes 



Fin. 607. Chiton, development. A, general view of larva ; 2?, section of early, and C, of later 

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

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

 mo. mouth ; rd. radular sac ; #j>. spines ; viac. g. visceral ganglion. (From Korschelt and 

 Heider, after Kowalewsky.) 



men 



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 

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

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

 is 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 

 on the shore between tidal limits. The Aplacophora, on the 

 other hand, are rare in very shallow water, and absent alto- 

 gether from the littoral zone: some have been found at con- 

 siderable depths (down to 1,250 fathoms). Some of them burrow 



