Glass 1. Placophora. 



291 



plateSj spines, and bristles, which cover the edge of the dorsal surface, 

 are true cuticular structures. The mantle is represented only by 

 a narrow fold which runs round the whole body, dorsal to the head 

 and foot (Fig. 268 A) ; it covers a series of plumose gills on either 

 side. The head, which is not very well marked, bears neither eyes 

 nor tentacles ; on the other hand, eyes occur irregularly on the 

 dorsal surface in several forms; they are situated at the tips of 

 the soft projections of skin which 

 perforate the shell-plates, and thus 

 apparently on the shell-plates them- 

 selves. The nervous system is 

 chiefly characterised by the fact that 

 two of the cords which spring from 

 the pedal ganglia and run posteriorly 

 (see p. 289) are very strong and are 

 connected by numerous transverse 

 cords. A well-developed radula 

 is present ; the anus is posterior 

 and median. The heart lies above 

 the rectum, it is possessed of two 

 auricles, arranged symmetrically on 

 either side of the ventricle. There 

 is a pair of elongate branched 

 kidneys, opening into the mantle- 

 groove, one on either side, just 

 anterior to the anus. The sexes 

 are separate ; ovary and testis single, 



but the ducts paired, and opening on either side a little in front of 

 the excretory pores in the mantle-groove. The 1 a r v se are oval, 

 provided with a velum, and two eyes which later undergo degeneration. 

 The smaller species of eyeless Chitons occur on British coasts ; the 

 larger forms live in the warmer seas. 



Formerly another small group of bilaterally symmetrical Mollusca was also 

 placed with the Gastropoda, namely, the Scaphopoda (genus Bentalium, 

 Elephants-tusks, etc.), in which the elongate body is surrounded by a slightly 

 curved, conical shell open at both ends. Further details of this gi'oup which is 

 in many respects yeiy aben-ant and isolated in position, cannot, however, be given 

 here. 



Mg. 238. Chiton, viewed from 

 below, somewhat diagrammatic. m 

 mouth, / foot, h gills, g genital pore, 

 71 urinary aperture, a anus. — Orig. 



Class 2. Gastropoda. 



The structure of the Gastropods is most easily comprehended on 

 the supposition that they have arisen by the modification of a Chiton 

 in the following way {cf., Fig. 268^ and Fig. 239). The dorsal 

 surface has become much arched, generally indeed drawn out into a 

 long sac, the ventral portion of which is surrounded by the lower 



V 2 



