COMPARATIVE ANATOMY 



CHAP. 



and new, morphologically different respiratory organs may be substituted. The 

 visceral dome may be flattened down, and the foot become rudimentary or disappear. 

 Teeth of all kinds may be wanting. The complex of the sub-pallial organs may be 

 so displaced as to lie anteriorly, thereby causing a very pronounced asymmetry of 

 the whole organism. But the typical Molluscan characteristics are never so entirely 

 obscured that the members of the race cannot be recognised, on the one hand by 

 means of transition forms leading to well-known Molluscan types, and on the other 

 by their developmental history. 



The Molluscs are divided into the five following classes : 



I. Amphineura. 

 III. Scaphopoda. 



II. Gastropoda. 

 IV. Lamellibranchia. 

 V. Cephalopoda. 



Systematic Review. 



CLASS I. Amphineura. 



Bilaterally-symmetrical Molluscs. The nervous system consists of two lateral 

 and two ventral nerve trunks, bound together by numerous commissures, and 



FIG. 1. Chiton, from life (after Pretre, in the Voyage de V'Astroldbe). 



provided with ganglion cells throughout their whole length ; these pass anteriorly 

 into the cerebral ganglion. Special sensory organs are reduced. Marine. 



ORDER 1. Placophora (Polyplacophora) sive Chitonidse. 



On the dorsal side there are eight consecutive shelly plates overlapping like the 

 tiles on a roof. There is a distinct snout. The branchiae are numerous, and are 

 arranged in two longitudinal rows, one on each side in the groove between the foot 

 and mantle. The foot (except in Chitonellus) is strongly developed, with a large flat 



