532 GASTEROPODA. 



consists of vascular filaments arranged in a pectinated 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. 



(1406.) It is from the position and arrangement of the branchial 

 organs that the branchiferous Gasteropoda have been classified by 

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

 NuB-iBRANcmATA, they are naked, and placed upon some part of the 

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

 but in others, as for example in Doris (fig. 266), 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. 



(1407.) In the INFEKOBRANCHIATA the branchiae resemble two long 

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

 edge formed by the mantle. 



(1408.) The TECTIBKANCHIATA 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 PleurobrancJius 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 (fig. 267, d). 



Fig. 267. 



Plcurobranchus. 



(1409.) But by far the most numerous order of the marine Gastero- 

 poda (PECTINIBRANCHTATA), which, in fact, includes all the inhabitants 

 of spiral univalve sea-shells, have their branchiae placed internally in 

 a capacious cavity, into which the water is freely admitted (fig. 262, a). 

 This cavity is situated in the last or widest turn of the shell, and com- 

 municates with the exterior of the body by a very wide slit, to which in 

 some genera a long siphon (fig. 262, /), formed by a fold of the mantle 



