270 THE VITAL FUNCTIONS. 



class, a beautiful gradation may be perceived : 

 the Branchiopoda having two hearts, one placed 

 upon each of the two lateral trunks of the 

 branoJiial veins ; the Gasteropoda having a single 

 heart, furnished with an auricle ; and the Ace- 

 j)hala being provided with a heart, which has 

 a single ventricle, but two auricles, corresponding 

 to the two trunks of the branchial veins.* 



The most remarkable variety of structure is 

 that exhibited by the Cephalopoda. We have 

 already seen, in the Crustacea, dilatations of the 

 vense cavae, at the origin of the branchial arte- 

 ries : but in the Nautilus the dilatations of the 

 branchial veins are of such a size, as to be almost 

 entitled to the appellation of auricles. The 

 Sepia, in whose highly organized system there is 

 required great additional power to propel the 

 blood with sufficient force through the gills, is 

 provided with a large and complicated branchial 

 apparatus ; and the requisite power is supplied 

 by two additional hearts, situated on the venae 

 cavag, of which they appear as if they were 

 dilatations, immediately before the branchial 

 arteries are sent ofF-t They are shown at e, e. 

 Fig. 355, which represents this part of the vas- 



* A great number of bivalve Mollusca exhibit the singular pe- 

 culiarity of the lower portion of the intestinal tube traversing 

 through the cavity of the heart. 



f These veins, are surrounded by a great number of blind 

 pouches, which have the appearance of a fringe ; the use of this 

 singular structure is unknov^n. 



