666 



ZOOLOGY 



SECT. 



and coiled. There are salivary glands and a large paired liver 



(Fig. 566, liv.) 



Body-Cavity. In the Aplacophora the interstices between 

 the organs and the body wall are rilled with a form of connective 

 tissue with muscular fibres; a vertical diaphragm (Fig. 565, dia.) 

 separates the posterior part of the body, containing the peri- 

 cardium (pen\ from the rest. In the Placophora the ccelome 

 ( Fig. 566), is an extensive cavity, lined with a coelomic epithelium, 

 and divided into three completely separated parts, the pericardium, 

 the genital cavity, and the general body cavity. 



Tbrn- 



mo 



-den. 



pert 



FIG. 565. Chsetoderma nitidulum, longitudinal section. <M. anus ; ln-n. brain ; o>r. gland- 

 ular cceca of mesenteron; dia. diaphragm separating off the posterior portion of the body 

 mo. mouth ; peri, pericardium and heart ; rail, radula ; rect. rectum. (After Simroth.) 



Vascular System. The existence of a heart in the Aplaco- 

 phora is somewhat doubtful ; the organ which is supposed to be 

 of that nature is contained in a cavity termed the pericardium 

 (Figs. 565 and 568, peri). 



In Chiton there is a well-developed heart (Fig. 566, lit.) consist- 

 ing of a median ventricle and two lateral auricles. The pericardial 

 cavity in which it lies is a space of considerable extent in the 

 posterior region of the body, below the two last valves of the 

 shell. 



The Nervous System consists in the Aplacophora (Fig. 567, 

 A,B, C) of four longitudinal nerve cords two pedal and twopallial. 



