CIRCULATION. 131 



artery which the heart sends off is immediately distributed to the two 

 sets of gills ; from these the blood passes out in two trunks, one on 

 each side, which after running a little way join into one vessel, as the 

 two aortas do in the turtle ; which now runs on as an artery to supply 

 the rest of the body with blood, without having first entered a heart, or 

 what is called the left auricle and ventricle. 



Thus the difference between the two classes is considerable ; but 

 nature, always proceeding by the nicest gradations, has formed two 

 animals which partake so much of the structure of the two classes, that 

 they gently lead us on from the one to the other. The first of these, as 

 being nearest to the amphibious tribe, are the animals now before us *, 

 which, indeed, form the next link in the chain that we are acquainted 

 with, as will be easily seen by comparing them together. The present have 

 but one auricle 2 and one ventricle, sending off one artery, which is com- 

 mon to the gills and lungs, and which might be called ' pneumobranchial.' 



Here is a falling off from the Amphibia of an auricle, and in some 

 measure of a ventricle, notwithstanding which, the effect of the heart 

 upon the blood is nearly the same. 



The artery passes out of the heart, sending off the pulmonary arteries, 

 which are ramified upon the lungs as usual, and then divides into two 

 branches, which are analogous to the two arteries in the turtle ; but as 

 these animals (Anvphiuma and Menopoma) are a degree nearer fish, 

 these arteries are each again subdivided into two, which afterwards 

 wind round those singular parts — in some measure similar to the gills 

 of fish — with which they are furnished. Having made this circuit, the 

 subdivided vessels again unite so far as to form only two trunks, and 

 these two presently join into one in the same manner as we have re- 

 marked in turtle and fish. 



The other animal — the siren — completes the gradation by being one 

 remove nearer to fish : in this the subdivision is not into two branches 

 only, as in that above described, but the whole aorta divides and sub- 

 divides into infinite ramifications, similar to the artery in the gills of 

 fish, while the lungs are supplied in the same manner as those of the 

 preceding animal and of the Amphibia. 



Thus the gradation is formed from perfect lungs, first to perfect lungs 

 and imperfect gills, then to perfect lungs and perfect gills, till at last 

 we have no lungs, but simply perfect gills, as in the fish. 



The animals of this class have but a single circulation, and of course 



1 [Amphiuma didactylum, Cuv., and Menopoma Alleghaniense, Harlan.] 



2 [I found this auricle divided by a complete septum in the Siren lacertina. — Trans. 

 Zoological Society, vol. i. p. 213 ; ' Animal Economy,' p .396, note. Prep. Mus. Coll. 

 Chir. No. 913a.] 



k 2 



