MOLLUSCA PTEROPODA. 257 



been accordingly comprised in the order of Mid- 

 tivalves, which also comprehends Cuvier's order 

 of Cirrhopoda, including the several kinds of JBar- 

 ncicles, (the genus Lepas of Linnaeus), which are 

 furnished with a great number of jointed fila- 

 ments, or cirrhi, and form an intermediate link 

 of connexion between the MoUusca and the Arti- 

 culata. But the limits of this treatise will not 

 allow me to dwell on the endless diversities of 

 structure which this subject presents. 



"^ 5. Pteropoda. 



In the MoUusca belonging to the two orders 

 which have now passed under our review, namely, 

 the Acephala and Gasteropoda, the mantle, while 

 it folds over the principal viscera of the body, 

 leaves apertures for the admission of water to the 

 gills, or organs of respiration. But there exist a 

 few genera having the sac formed by the mantle 

 closed on every side ; a structure which renders 

 it necessary to adopt a different arrangement 

 with regard to the gills, and to place them ex- 

 ternally, and we then find them spreading out 

 like a pair of wings, on each side of the neck. 

 Since this general closing of the mantle pre- 

 cludes, also, the formation of any organ of pro- 

 gressive motion corresponding to a foot, advan- 

 tage is taken of the projection of the gills to em- 



VOL. I. s 



