1909. } MECHANISMS IN FISHES AND A SNAKE. 203 
forms a closely adherent lining to a continuous cartilaginous 
canal. 
In any case the aorta is to a great extent cut off from the 
direct effects of those internal movements and pressures to which 
in Fishes the arterial circulation is probably to a large extent due, 
as in the veins of Mammals. 
We thus see in the case of these particular fishes the advantage 
of some special mechanism to aid the circulation of the blood. 
Text-fig. 21. 
WANE 
Diagrams of one and the same portion of the aorta, showing the relative positions 
of the ventral ligament and aortic cavity during the lateral flexions of the body 
in swimming. 
The ligamentum longitudinale is apparently formed around, 
though not actually from, the subchordal rod *—a structure that 
occurs as a transitory foetal organ in Fishes and Amphibia, and 
from its mode of development appears to be the vestige of the 
epipharyngeal groove of Amphioxus. 
* Franz, Morph. Jakrb. Bd. 25, 1897. 
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