446 C. H. DANFORTH 



rounded part below rests on the ventral inside wall of the aorta, 

 thereby dividing it into essentially distinct longitudinal com- 

 partments. In specimens fixed in a curved position the Hgament 

 stretched diagonally from side to side in accordance with the 

 bends in the body. Perhaps, in addition to the function sus- 

 gested by this condition, it may also act in opposition to the 

 interspinous ligament. This peculiar structure was also observed 

 by Bridge (79), who suggested tentatively that it might repre- 

 sent the subchordal rod of elasmobranchs. I find a similar 

 structure occuring in Scaphirhynchus and a rudiment of it in 

 the trout and Ameiurus. 



The branches of the dorsal aorta are an irregular series of 

 segmental arteries, beginning with the subclavian in front and 

 given off throughout the whole course; a series of short ventral 

 branches to the air-bladder, gonad, and kidney; and the large 

 coeliaco-mesenteric artery which arises at the anterior extremity 

 of the coelom. The aorta itself terminates as it approaches 

 the end of the tail by dividing into short lateral branches which 

 pass out of the axial cartilage and again divide, this time into 

 dorsal and ventral branches for the two lobes of the tail. 



THE SEGMENTAL ARTERIES 



There are two main divisions of the segmental arteries, the 

 dorsal, or parietal, the ventral, which in the coelomic region 

 are the splanchnic arteries. They may arise along the lateral 

 ti'iangle of the aorta by short common stems or, more frequentl}^ 

 the dorsal and ventral divisions are quite distinct from each 

 other. These vessels are somewhat irregular in their appear- 

 ance and vary greatly in size. For the most part, however, 

 there is one pair of good sized arteries for every two or three pair 

 of nerves. Sometimes the two members of a pair will be very 

 different in size and sometimes two successive arteries on the 

 same side will both be large, especially in the region of a fin. 

 Each dorsal parietal artery (fig. 18, a. pa.), on emerging from 

 the aortic wall, sends a large horizontal branch diagonally out- 



