474 PHILOSOPHY OF ZOOLOGY. 



preceding genera. In some, the branchiae form a complete 

 circle ; in others, the circle is interrupted anteriorly at the 

 head. 



Within the trunk, the mouth is fortified by two cartila- 

 ginous cheeks, which, at their union anteriorly, support 

 the base of the tongue. This last is a most singular organ. 

 It is longer than the whole body, narrow, and covered with 

 three rows of short reflected spines, interrupted longitudi- 

 nally and transversely. Its fixed end only can be exercised in 

 deglutition, its free end being coiled up the abdomen. On the 

 upper side of the mouth is a semicircular osseous plate, or 

 upper jaw. The gullet is furnished with a dilatable pha- 

 rynx. The stomach is elliptical, with the cardia and pylorus 

 at opposite extremities. The intestines are variously fold^ 

 ed, and are several times longer than the body. The saliva- 

 ry glands are minute. The liver is intimately united with 

 the stomach and intestines. 



The heart is situate on the left side, in the anterior 

 part of the body. The auricle receives the aerated blood 

 from one vein when the circle of the gills is complete, and 

 by two when interrupted. This auricle is placed on the 

 anterior side of the heart. An aorta arises from each side, 

 to convey the blood to the body. 



The ovarium is placed underneath the liver ; and, as it 

 exhibits some differences of organization, M. CUVIER infers 

 that it likewise contains the male organs. 



The species belonging to this genus are numerous, and 

 appear to admit of distribution into sections ; the first hav- 

 ing the branchial circle complete, the second interrupted *. 



b. Shell divided. Mouth destitute of tentacula. 



* See a figure of P. vulgaris, Plate IV. f. 3. 



