534 GASTEROPODA. 



CYCLOBRANCHIATA, because the branchiae form a fringe around the body 

 of the animal, between the edge of the body and the foot (fig. 260, c ; 



fig. 263, <>: 



(1414.) Lastly, a distinct order has been established to embrace 

 certain families in which the foot is so much compressed as to constitute 

 a vertical muscular lamella, that presents merely a remnant of the 

 ventral sucker so characteristic of the entire class, and which can only 

 be serviceable in performing the office of a fin used in swimming ; hence 

 these mollusks have been called HETEROPODA. Their branchiae are 

 placed upon the back (fig. 269, d), and resemble small detached tufts. 

 The form of these heteropod Gasteropoda the reader will gather from 



Fig. 269. 



Pterotrachea. 



an inspection of the accompanying figure, representing a species of 

 Pterotrachea ; but the details connected with their anatomy, therein 

 delineated, will be explained hereafter. 



(1415.) It would be useless to weary the student by describing the 

 course of the blood-vessels in all the orders we have just enumerated ; 

 their distribution necessarily varies with the changes observable in the 

 position of the branchiaB ; still, whatever the situation of the respiratory 

 organs, the general course of the circulation is the same, and essentially 

 similar to what has been already described in the Snail : one or two 

 examples will therefore answer our purpose. In the Pectinibranchiata, 

 as for instance in Buccinum (fig. 275), the heart (r, s), enveloped in a 

 distinct pericardium, is placed at the posterior extremity of the branchial 

 chamber, and consists, as in all the GASTEROPODA, of two cavities a 

 thin membranous auricle, and a more muscular and powerful ventricle. 

 It receives the blood from the organs of respiration by a large branchial 

 vein (fig. 275, q), that communicates with the auricle (a). The con- 



