PARKER AND DAVIS : HEART BLOOD VESSELS IN PISHES. 175 
larity between the two sets of structures. Vessels of Thebesius 
may open into the single auricle and ventricle of a fish as they open 
into the paired auricles and ventricles of a mammal; and their 
freer communication with the coronary veins than with the coro¬ 
nary arteries in the fish recalls the condition found in mammals. 
Morphologically the vessels of Thebesius in fishes seem to corre¬ 
spond exactly to those in mammals. 
The superficial veins of the heart in fishes are also much like 
those in mammals. A right, a left, and a median system can be 
distinguished, and in Carcharias the vessels representing these are 
almost identical with those in the mammalian heart. In only one 
important respect do the superficial veins in fishes differ from those 
in mammals; in fishes these veins open into the venous sinus, in 
mammals into the right auricle. When, however, it is remembered 
that the venous sinus in mammals becomes incorporated in the 
right auricle this supposed difference disappears. There is then no 
reason for supposing that the veins of the mammalian heart are 
not homoloo;ous with those in the heart of the fish. 
The coronary arteries in fishes show less resemblance to those 
in mammals than has been noticed between the other classes of 
vessels, and this is particularly true of the way in which the coro¬ 
nary arteries originate. These arteries in mammals arise from the 
base of the aortic arch very near the heart; in fishes they come 
from the efferent branchial arteries at places that would correspond 
to positions well towards the dorsal side of the aortic arch of a 
mammal. The fact that the ventral aorta of a fish carries impure 
blood and the corresponding vessels in a mammal pure blood, is a 
sufficient physiological reason for this difference, but it leaves the 
question of the homology of these parts entirely open. Have the 
coronary arteries of mammals been derived from those of fishes or 
are they a new system of vessels ? The supposed absence of coro¬ 
nary arteries from the heart of amphibians has been urged in favor 
of the latter opinion, but Martin’s (’94, p. 59-60) statement that 
in the tadpole a. system of coronary arteries essentially like that in 
the fish is replaced towards the close of larval life by other coro¬ 
nary arteries is in reality almost the first piece of positive evidence 
bearing on this question. While this evidence is opposed to the 
homology of the coronary arteries of higher and lower vertebrates, 
it must not be forgotten that the vessels of Thebesius and the 
coronary veins of the higher and lower forms show every evidence 
