GASTEROPODA. 405 



In the INFEROBRANCHIATA the branchiae resemble two long 

 rows of leaflets, placed on the two sides of the body, under a 

 projecting edge formed by the mantle. 



The TECTIBRANCHIATA have respiratory organs upon one side 

 of the body only, and concealed by a flap derived from the 

 mantle. Such, for instance, is the case with Pleurobranchus and 

 Aplysia ; in the former of which the elegant branchial fringe is 

 situated in a deep sulcus between the edge of the mantle and 

 the prominent margin of the foot (Jig. 186, d). 



But by far the most numerous order of the marine Gasteropoda, 

 (PECTINIBRANCHIATA,) which, in fact, includes all the inhabit- 

 ants of spiral univalve sea-shells, have their branchiae placed inter- 

 nally in a capacious cavity, whereinto the water is freely admitted 

 (Jig. 196, a). This cavity is situated in the last or widest turn of 



Fig. 186. 



the shell, and communicates with the exterior of the body by 

 a very wide slit, to which in some genera a long syphon (Jig. 

 196, /), formed by a fold of the mantle or general covering of the 

 animal, conducts the respired fluid. The branchiae themselves, 

 as the name of the order indicates, are pectinated and form a 

 single, double, or triple series of gills suspended from the roof 

 of the branchial chamber, answering the same intention as the 

 pulmonary net-work of the snail, but deriving their supply of air 

 from the water, in which they are perpetually immersed. In the 

 figure referred to, representing a species of Pterocera, the position 

 of the branchial chamber is seen through the shell and mantle, 

 which the reader must suppose to be transparent; and the 

 branchial organ (a), in this case single, is likewise represented 

 in situ, suspended from the roof of the cavity that contains it. 

 In Jig. 198, the roof of the respiratory cavity (x) has been 



