PHYLUM MOLLUSCA 



283 



ficial layer of the shell valves. The megalsesthetes may take the form 

 of eyes, with cornea, lens, pigment layer with iris, and retina. 



There are two symmetrical nephridia (Fig. 170) opening internally 

 into the pericardium by a ciliated funnel-like opening (n. fieri, afi), and 



clen 



€Z7L 



Fig. 170. — Chiton, nephridial and genital systems, an, anus; cten, ctenidia; gen. 

 ap, genital aperture; gon, gonad; gouod, gonoduct; mo, mouth; neph. ap, 

 nephridial aperture; n.peri. ap, aperture from nephridia to pericardium. (From 

 Simroth, after Haller and Lang.) 



opening on the exterior (jieph. af) y 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, 



