CRVPTOCHITON STELLERI MIDD. 



215 



At the extreme lateral border of the percardium and about one 

 fourth its greatest length from its anterior end the relatively large 

 inner opening occurs (Fig. 2, r) and almost immediately leads 

 into a flat disc-like cavity closely attached to the ventral pericar- 

 dial wall. From the inner border of this flattened sac a slender 

 delicate tube, usually almost invisible in preserved material, pro- 

 ceeds forward and becomes continuous with the more dorsal 

 tube of the glandular portion of the kidney proper. This last- 

 named section consists, as in chitons 

 generally, of numerous highly branched 

 lobules which extend ventrally some 

 distance on the inner surface of the 

 foot, dorsally to the valves of the 

 shell and to the head cavity in front. 



In a number of species of chitons 

 the outer limb of the excretory canal 

 expands into a well defined reservoir 

 (Nierensack), which is also supplied 

 with glandular outgrowths. In some 

 cases these reservoirs are of compara- 

 tively large size and in a few cases they 

 may extend close to the mid-line, but 

 here, as in every species of chiton 

 hitherto described, one kidney is 

 wholly independent of the other. In 

 Ciyptochiton, on the other hand, the 

 reservoirs, that are probably distinct 

 in very young specimens, unite in the 

 mid-line and form, as Figs. I and 2 

 show a spacious chamber lying ventral to the anterior third of the 

 pericardial cavity. In specimens one fourth the adult size, and 

 occasionally in individuals of much greater length, there are 

 slight indications of a median partition that may represent the 

 line of fusion of the once independent divisions. 



Carefully removing the rectum and opening the ventral wall 

 of this common reservoir the latter will be found to terminate 

 posteriorly in two triangular diverticula, each of which communi- 

 cates by means of a narrow slit immediately behind the trans- 



Fig. I. Diagram illustrating 

 the kidney of C. stelleri, ventral 

 view ; p, pericardium. 



