L AMPHINEURA. 



357 



mantle extends beyond the shell and is covered with spines, while 

 in the mantle cavity beneath are, right and left, a series of ctenidia. 

 Nerves enter the shell and end with noticeable sense organs (aes- 



FiG.344. Chiton squamosus, dorsal view. (After Haller.) A, the entire animal; /?, 

 after removal of shell and viscera, a, anus; C', brain; K, ctenidia; o, mouth; 

 P, pedal nerve cord; pi, pleurovisceral nerve cord. 



thetes and, in some, eyes, fig. 345). The symmetry of the body is 

 also expressed in the viscera. The anus is medial, and right and 



FIG. 345. Eye and aesthetes of Acanthopleura spiniger. (After Moseley.) a, macrges- 

 thete; b, micrassthete; /, calcareous cornea; gr, lens; /i, iris; fc, pigmented cap- 

 sule ; n, p, nerves ; ?, retina. 



left of it are the openings of the nephridia and sexual organs. The 

 sexes are separate, the gonads unpaired, while corresponding to the 

 paired arrangement of the gills there are two auricles to the heart. 



The Chitons are represented on our northeastern coast by several 

 small species (Trachydermon,* Auricula*}; farther south and on the 

 Pacific shores are larger species (Cryptochiton *). 



