CHAPTER XL. 



POLYPLACOPHORA AND SCAPHOPODA. 



Class III. Polyplacophora. 



The class of the Polyplacophora comprises only the family of the 

 Chitonidce, which differs in many important respects from the Gastro- 

 pods, and presents certain relationships with the Worms. The body 

 in the Chitonidce, (fig. 731, a) is elongated and worm-like, bilaterally 



symmetrical, with the mouth 

 I at the anterior end, and the 



anus at the hinder extremity 

 of the body. There is no 

 differentiated head, nor are 

 cephalic tentacles develop- 

 ed. The under surface of 

 the foot forms a creeping 

 disc ; and the gills are nu- 

 merous (fig. 7 3 1 , £"), and are 

 contained in an imperfect 

 pallial chamber or groove, 

 between the margins of the 

 foot and the edge of the 

 mantle. The upper surface 

 of the body is covered by 

 the mantle, which secretes 

 a " multivalve " shell (fig. 

 731, b), composed of eight transverse imbricated plates, which suc- 

 ceed one another from before backwards, and are embedded in the 

 leathery or fibrous border of the mantle. 



The shell of the Chitons (fig. 731, b, and fig. 732) consists of 

 eight valves placed in a line, the anterior or " cephalic " valve and 

 the posterior or " anal " valve invariably differing from the six " in- 

 termediate " valves, which are similar to one another. Each of the 

 intermediate valves is divisible by lines of sculpturing into a central 



A B 



Fig. 731. — a, Under surface of a species of Chiton 

 (after Cuvier) : t, Fringe of tentacles round the mouth 

 (o) ; g, Branchiae ; _/, Under surface of foot ; a, Anus. 

 b, Shell of Chiton squamosus, reduced one-half. 

 (After S. P. Woodward.) 



