1891 ] The. Evolution of the Circulatory Organs. 237 
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THE EVOLUTION OF THE CIRCULATORY ORGANS, 
BY W. C. CAHALL. 
I” the March (1890) number of the AMERICAN NATURALIST I 
attempted to marshal the evidences furnished by the teeth in 
support of the hypothesis of evolution. Any other organ or 
group of organs could have been selected and found equally rich 
in evidences of a similar import. But none, perhaps, approach 
quite so nearly to a demonstration, as the beautiful series of 
cardiac organs met with by the comparative anatomist in his 
study of zoology. 
Origin. of Circulatory Organs—In the lower forms of life, the 
Protozoa, where no differentiation of structure has yet taken 
place, the organs of circulation, like those of. digestion, are not 
needed, for every part of the organism performs its own act of 
digestion and absorption of nutriment. Where a digestive tube 
is formed, as in the Hydra, the digested food passes by direct 
absorption into the tissues of the body. In the same group 
with the Hydra, the Ccelenterata, there are species where from 
the digestive tube radiate numerous canals which distribute the 
chyme to every part of the body. 
In other species of the same group we see slight but signifi- 
cant and progressive changes in these canals. This “ gastro- 
vascular system,” as it has been called, is the first approach to 
circulation we meet with in ascending the animal scale. It is for 
all practical purposes an efficient circulatory system, yet it is, 
structurally, nothing more than an amplification of the digestive 
tract. 7 
The first approach to independence of the organs of circulation 
from those of digestion is within the Vermes or worms. They 
have walls and are blood-vessels indeed, since they have a regu- 
lar circulation of a blood-fluid. A simpler kind is that of some 
Nemertina, where the main trunks are three long canals con- 
nected by transverse shorter ones (Fig. 1). 
In Fig. 2 is represented a more complicated system as found 
