282 



MANUAL OF ZOOLOGY 



The conspicuous organs of special sense present on the head of 

 Gastropods (see p. 284) are absent in the Chitons. A pair of 

 processes situated in front at the sides of the mouth have the character 

 of labial palps. Remarkable sensory organs, the micrasthetes and the 

 megalaslhcles, lie in the canals already mentioned as occurring in the 

 superficial layer of the shell valves. The megaloesthetes may take the 

 form of eyes, with a complex structure. 



There are two symmetrical nephridia (Fig. 158) each opening 

 internally into the pericardium by a ciliated funnel-like aperture (, 



mo 



pip 



manl- 



clen. 



FIG. 157. -Chiton, ventral view. an. anus ; cicn. ctenidia \ft. foot ; mant. mantle 

 edge ; mo. mouth. (After Pelseneer.) 



peri, ap.), and externally (neph. aft.) between two of the posterior 

 ctenidia. Each consists of a looped main tube, into which open 

 numerous minute tubules which ramify among the viscera. The sexes 

 are distinct. The testis and ovary (gon.) are similar in appearance, 

 differing only in colour when the products are mature. Each is an un- 

 paired sac marked by a series of slight lateral constrictions. The larva is 

 a Trochosphere. 



All the Amphineura are marine. The Placophora (Chitons) occur 

 at all depths, though most abundant on the shores between tidal 

 limits. The shell-less forms (Aplacophora), on the other hand, are 



