PERENNIBRANCHIATA. 38 



The third order of animals are those commonly called Amphibia : in 

 these the circulation is twofold, although not so distinctly as in the 

 next, because the two circulations become blended in the heart. Their 

 hearts consist of two distinct cavities, which are the two auricles, and 

 of two ventricles ; but the ventricles communicate so freely with one 

 another that they are to be considered as only one cavity ; therefore 

 these may be called Tricoilia. There the blood from the lungs, and 

 that which has gone through the other parts of the body, mix together, 

 instead of being separated, as in the more perfect animals ; so that 

 some of the last sort [venous] is thrown back through the body again 

 without passing previously through the lungs, and some of the first 

 sort is pushed a second and perhaps a third time through the lungs, 

 without being first employed in the general circulation. 



The last division comprehends the most perfect animals, which have 

 a double circulation, one through the lungs, the other through the 

 whole body, and for that purpose are furnished with a double heart. 

 The two auricles and two ventricles of which they are composed make 

 up the four cavities by which they are distinguished, and they may be 

 called Tetracoilia, 



This general account being premised, it is to be now observed, that 

 the tribe of animals before us are an intermediate link between the 

 second and third of the divisions above mentioned, by which they are 

 connected with, or, as it were, run into, each other. For 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 1 , 

 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 common to the gills and lungs, and which might be called ' pneu- 

 mobranchial.' 



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 arte- 

 ries, 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 



1 [Amphiuma didaetylum, Cuv., and Menopoma Alkghaniensis, Harlan.] 



2 [I found this auricle divided by a complete septum : see Transactions of the 

 Zoological Society, vol. i. p. 213. ' Animal Economy,' 8ro. p. 396, note.] 



VOL. II. 2 c 



