270 THE VITAL FUNCTIONS. 



organ in the several orders of this class ; the 

 JBrancMopoda having two hearts, one placed upon 

 each of the two lateral trunks of the branchial 

 veins ; the Gasteropoda having a single heart, 

 furnished with an auricle ; and the Acephala 

 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 

 venae 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 

 cavse, 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. 



t 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 unknown. 



