MOLLUSCA. 413 



are few in number, and form a very natural tribe, which 

 CuviER has termed gasteropodes pulmwnes. In them, the 

 respiratory organ is simple, consisting of a shiglc cavity, on 

 the walls of which the extremities of the pulmonary artery 

 are spread. This cavity communicates externally by an 

 aperture which the animal can open or shut at pleasure. 



The mollusca which breathe by means of gills, exhibit 

 very remarkable differences, in their number, structure, 

 and position. In some cases, there is a single cavity com- 

 municating by an aperture, through which the water enters. 

 The walls of this cavity exhibit an uneven surface, dispos- 

 ed in ridges, which are the gills, and on which the pulmo- 

 nic artery is expanded. This structure exhibits itself in 

 the Gasteropoda pectini-branchia. In many cases the giUs, 

 though seated in a cavity, like the former, and equally ex- 

 posed to the contact of the surrounding element, are two 

 in number, one on each side, as in the Cephalopoda. In 

 the Bivalvia, they are four in number, two on each side 

 like leaves, and extend the whole length of the body. 

 In these, the water is admitted at the pleasure of the ani- 

 mal. 



The gills of other mollusca are seated externally, and 

 consist either of arborescent productions, or simple cuticu- 

 lar elongations, within which the pulmonary artery termi- 

 nates. In some of these, as the Pteropoda, the branchial 

 surface is constantly exposed to the action of the surround- 

 ing water ; while in others, the cuticular expansions, which 

 axe analogous to gills, are retractile at the will of the ani- 

 mal. 



By means of the characters furnished by the circulating 

 and respiratory systems, the molluscous animals may be 

 divided into several distinct classes. But as we shall em- 

 ploy these characters in the construction of the different 



