GASTEROPODA. 407 



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

 ventral sucker, so characteristic of the entire class, and \vhich can 

 only be serviceable in performing the office of a fin used in swim- 

 ming ; hence these mollusks have been called HETEROPODA. 

 Their branchise are placed upon the back (Jig- 188, d), and 

 resemble small detached tufts. The form of these heteropod Gas- 

 teropoda the reader will gather from an inspection of the accom- 

 panying figure, representing a species of Pterotrachea ; but the 

 details connected with their anatomy therein delineated, will be 

 explained hereafter. 



Fig. 188. 



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

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

 rated ; their distribution necessarily varies with the changes ob- 

 servable in the position of the branchise ; 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 pur- 

 pose. In the Pectimbranchiata, as for instance in Buccinum (fig. 

 193), 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 

 (Jig. 193, q), that communicates with the auricle (s). The con- 

 traction of the auricle forces the circulating fluid into the ventricle 



