1!) 



Class I. AMPHINEURA. 



(Cases 1 to 3.) 



These Mollusca arc elongate and bilaterally symmetrical, with 

 the head and anus at opposite ends of the body. They are all 

 marine animals, and some of them are found at great depths. 

 Spicules are present in the mantle. The Amphineura are divided 

 into two orders, the Polyplacophora and the Aplacophora. 



The Polyplacophora include the ' Coat-of-mail ' Shells (Chito- 

 nidae). They are characterized by the possession of a series of 



Fig 



Coat-of-mail Shells, or Chitons. 



1. Chiton squamosus (upper surface). 



2. Chiton elegans (lower surface) : a. mouth ; b. foot 



c. mantle ; d. gills. 



eight plate-like shells on the back, typically well developed and 

 overlapping (e.g. Chiton) but in some genera (e.g. Cryptoplax, 

 Schizochiton) reduced and well separated. In many forms the 

 shells are covered over by the mantle, as in the giant Cryptocliiton 

 stelleri of the North Pacific. The shell is traversed by a number 

 of fine branching canals in which are lodged peculiar sense-organs 

 known as ' shell eyes ' ; these may have a retina and lens. The 

 upper surface of the mantle may be smooth, granular, or armed 

 with spicules. Between the foot and the mantle is a row of gills 

 which may be nearly continuous round the body. The heart is 

 dorsal in position and is very elongate. 



Chitons are found in all parts of the world. They live chiefly 

 on rocks and stones in pools near low tide-mark, but a few have 



B 2 



