446 PHILOSOl'HY OF ZOOI-OY. 



sulcated transversely, and the other but slightly so.*" — 

 Tuckei/s Narrative^ p. 412. 



B. Fm single. 



n. Cymbulia — Tunic cartilaginous and trough-shap- 

 ed. 



The fin is single, divided into three lobes, one of which 

 is small, with two tubercles, and a minute fleshy beard. 

 This genus was instituted by Peron, in Annates du Mu- 

 seum, t. XV. t. 3. f. 10. 11. 



Section 2. — Gasteropoda. 

 Organs of Progressive Motion fitted Jbr Creeping. 



This is one of the most extensive groups of Molluscous 

 animals. The marks by which it is distinguished are well 

 defined, and the external and internal characters of the spe- 

 cies have been successfully illustrated. 



The gasteropoda may be considered as having the body 

 protected dorsally by the cloak, and ventrally by the foot. 

 The cloak is either continuous, and usually more or less 

 arched, for the reception of the viscera underneath, or it is 

 interrupted by a projecting bag, in which are contained the 

 principal digestive and reproductive organs. This project- 

 ing bag is tapering and spiral, and always protected exter- 

 nally by a shell. When the cloak is continuous, the sur- 

 face is variously marked, and frequently exhibits a parti- 

 cular portion, more elevated than the rest, in some cases 

 concealing a testaceous plate, which has been termed the 

 shield. 



l^hejbot situate on the ventral surface, and in opposition 

 to the cloak, exhibits a flat, soft surface, consisting of inter- 

 laced muscular fibres. Its central surface serves as a sup- 

 port to the viscera, while externally it constitutes the organ 



