THE HEART AND ARTERIES OF POLYODON 445 



It will be seen from the foregoing account that the relations 

 of the internal carotid are rather more in accord with conditions 

 in such other ganoids as Amia and Acipenser than is the case 

 with the external carotid. The connection of the internal caro- 

 tid with the efferent pseudobranchial and ophthalmica magna 

 arteries is clearly explained in Allis's paper, and calls for no 

 farther discussion here. It remains only to be said that in my 

 larva, which is younger than those studied by Allis, the relative 

 size of the internal carotid and the efferent pseudobranchial 

 artery running beside it is very different from what it is in the 

 adult. In a 74 mm. specimen the internal carotid, instead of 

 being much the smaller vessel, is five or six times larger than 

 the other. This would seem to indicate that in Polyodon the 

 presence of an anterior carotid (i.e., one deriving its supply 

 chiefly from the pseudobranch) is secondary and not the primary 

 condition as it is said to be in sharks. 



THE DORSAL AORTA 



The origin of the dorsal aorta from the efferent branchial 

 arteries has already been described. A few millimeters behind 

 the entrance of the last pair of arteries the aortic wall becomes 

 chondrified and the vessel from this point on is invested by a 

 thick wall of cartilage which is interrupted only in the mid- 

 dorsal line. It is decidedly flattened dorso-ventrally, so that 

 in cross section it is in the form of a small sector of a large circle 

 or even crescentic in outline. In surface view, when dissected 

 out, the dorsal aspect shows regular metameric markings in 

 the cartilage, but the ventral aspect is ridged and furrowed in 

 a very irregular manner. A most striking feature in connection 

 with the vessel is a longitudinal ligament (fig. 18, lig.), suspended 

 in the lumen throughout its entire length. This ligament is 

 attached in front and behind and supported throughout by a 

 continuous thin membrane of fibrous tissue which binds it to 

 the notochordal sheath along the non-chondrified dorsal line of 

 the aortic wall. The flat suspensory portion is so deep that the 



JOURNAL OF MORPHOLOGY VOL. 23, NO. 3 



