MOLLUSCA. 475 



42. CHITON. Shell constituting a series of imbricated 

 dorsal plates. 



The body is elliptical. The cloak is firm and cartila- 

 ginous, and variously marked on the margin. The dorsal 

 plates are arched, and occupy the middle and sides of the 

 back, where they are implanted in the cloak, in an imbri- 

 cated manner, the posterior margin of the first valve covering 

 the anterior margin of the second. The foot is narrow. The 

 mouth is surrounded with a semicircular curled membrane, 

 and is destitute of tentacula. The anus consists of a short 

 tube, placed at the posterior extremity of the cloak. The 

 external orifice of generation has not been detected. 



The mouth is capable of forming a short proboscis. The 

 tongue is short, and armed with strong, reflected spines. 

 The gullet is short, and the stomach, which is lengthened 

 and folded, is membranaceous. The intestine is several 

 times longer than the body, and much folded. The liver 

 is divided into numerous lobes, and intimately united with 

 the stomach and intestines. 



The heart is situate at the posterior part of the body. 

 The auricle is placed posteriorly, and receives the aerated 

 blood from two veins. Each vein descends along the base 

 of the gills, collecting the aerated blood from the particular 

 side of the body to which it belongs; and, what is most re- 

 markable, when opposite the ventricle, it is suddenly en- 

 larged, and sends off a branch which communicates with it, 

 and again contracts and unites with its fellow from the op- 

 posite side, to form the auricle. A single aorta arises from 

 the anterior side. 



The ovarium is conical, and divided into numerous lobes. 

 Behind, two ducts seem to arise, and to proceed one to 

 each side ; but it has not been determined whether they 

 open externally. No male organs have been detected ; nor 



