230 THE MECHANICAL FUNCTIONS. 



which also comprehends Cuvier's order of Cirrho- 

 poda, including the several kinds of Barnacles (the 

 genus Lepas of Linnseus), which are furnished with 

 a great number of jointed filaments, or cirrhi, and 

 form an intermediate link of connexion between 

 the Mollnsca and the Articidala* 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 Mollusca 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 neces- 

 sary to adopt a different arrangement with regard 

 to the gills, and to place them externally ; and we 

 then find them spreading out like a pair of wings, 

 on each side of the neck. Since this general clos- 

 ing of the mantle precludes, also, the formation of 

 any organ of progressive motion corresponding to a 

 foot, advantage is taken of the projection of the 

 gills to employ them as oars for the purpose of en- 

 abling the animal to swim through the water. 



* The Cirrhopoda are now by most naturalists removed from the 

 class Mollusca, and transferred to that of Crustacea. Mr. J. V. 

 Thompson has discovered that they undergo distinct metamorphoses. 

 Zoological Researches, 1830. 



