IN THE PERENNIBRANCHIATE BATRACHIA. 217 
In Amphiuma and Menopoma the contracted membranous part of the aorta intervening 
between the ventricle and bulb, is proportionately longer than in the Siren, but has the 
same spiral twist. The bulb itself, on the other hand, is much shorter, and broader : 
the ventricle in both these genera is also shorter in proportion to its breadth, and in 
Menopoma approximates to the triangular form which characterizes the ventricle iu 
Osseous Fishes. This genus also presents an affinity to Cartilaginous Fishes, in haying 
two rows of semilunar valves in the bulbus arteriosus, three in each row ; but neither 
in Menopoma nor Amphiuma is there any fleshy cylindrical body in the bulb,—this ad- 
ditional valve being unnecessary from the free passage which the undivided branchial 
arteries afford to the blood in these genera. In Amphiuma the pulmonary arteries are 
given off from the end of the bulb of the branchial aorta: in Menopoma they are formed 
by the union of two twigs given off respectively by the first and second branchial arte- 
ries near their origin, In the Siren the pulmonary arteries are branches of the lower 
branchial vein. It is worthy of observation, that in both Amphiwma and Menopoma 
the pulmonary artery supplies other parts besides the lungs, branches being sent off 
from it to the wsophagus: but I have not been able to trace any ramifications to the 
skin, as has been observed by Dr. Davy to be the case in the Toad. 
The presence of two auricles in the heart of the Reptiles doutewx now renders appli- 
cable to the whole class of Reptiles the phrase ‘‘ Cor uniloculare biauritum”’ ; and forms 
an additional argument for retaining as an Order of that Class the Amphibia of Latreille. 
But besides the zoological application of the preceding anatomical facts, they are in- 
teresting also in a physiological point of view. 
From the impediments which frequently occur to a free and regular circulation of 
blood in these cold-blooded and slow-breathing animals, the venous side of the heart is 
subject to great distension ; hence the large size of the auricles, and of the sinus which 
receives the systemic veins, and the perfect development of the intervening pair of 
valves, of which the Eustachian valve in the Mammiferous heart still presents a rudi- 
ment. Had the pulmonary veins terminated along with the systemic in the same 
cavity, their orifices would have been subjected to the pressure of the accumulated 
contents of that cavity, and there would have been a disproportionate obstacle to the 
passage of the aerated blood into the ventricle. This is obviated by providing the pul- 
monary veins with a distinct receptacle, which is equally ready with the right auricle 
to render its contents into the ventricle during the diastole of that cavity. 
In considering the heart with reference to the breathing organ in other classes, we 
find that at its first appearance as a distinct mechanical and muscular organ its energies 
are expended on the systemic circulation, and that the respiratory apparatus is placed 
at the termination of the circle ; the venous blood, prior to re-entering the heart, being 
either diffused in extensive and irregular sinuses, over whose parietes air is distributed 
by minutely ramified trachee, as in Insects ; or passing from venous trunks to branches 
indefinitely ramified upon a more concentrated respiratory organ, as in Crustaceans and 
