404 



GASTEROPODA. 



order to breathe, to come continually to the surface of the shal- 

 low pools wherein they are found. The Planorbis and Limnceus 

 are examples of this mode of respiration ; and are met with in 

 every ditch, where they voraciously devour the subaquatic vege- 

 tables upon which they feed. 



(441.) It is at once evident that in marine Gasteropods another 

 mode of aerating the blood must be resorted to, and branchiae of 

 some description or other pig. 185. 



substituted for a pulmo- 

 nary cavity. 



The branchiae given 

 for this purpose are 

 variously constructed ; 

 sometimes appearing as 

 extensively branched 

 and arborescent appen- 

 dages to the skin, or 

 else they form broad 

 and thin lamellae at- 

 tached to the exterior 

 of the body ; but more 

 frequently the respi- 

 ratory apparatus con- 

 sists of vascular fila- 

 ments arranged in a pec- 

 tinated manner along a 

 central stem : whatever 

 their form, however, 

 their office is the same, 

 namely, to present a 

 sufficient surface to the 



surrounding medium, in order adequately to expose the blood that 

 circulates abundantly through them to the influence of oxygen. 



It is from the position and arrangement of the branchial organs 

 that the branchiferous Gasteropoda have been classified by zoolo- 

 gists. Thus in the second order, called from this circumstance 

 NUDIBRANCHIATA, they are naked and placed upon some part of 

 the back ; sometimes, as in Tritonia, extending along its entire 

 length; but at others, as for example in Doris (Jig- 185), they 

 are confined to its posterior part, and form a circle around the anal 

 orifice of exquisite beauty, and not inaptly comparable to a flower 

 in appearance and disposition. 



