PHYLUM MOLLUSCA 



283 



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

 of eyes, with cornea, lenSj^pigment layer with iris, and retina. 



There are two symmetrical nephridia (P'ig. 170) opening internally 

 into the pericardium by a ciliated funnel-like opening {n.peri. ap), and 



-gonad 



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

 ap, genital aperture; gon, gonad; gonad, 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 {neph. ap'), 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, 



