416 C. H. DANFORTH 



end. They are disposed in three pairs, morphologically com- 

 parable to similar vessels in elasmobranchs. The most anterior, 

 resulting from a terminal bifurcation of the aorta, is a short 

 trunk on either side, which presently divides to form afferent 

 hyoidean (ahy.) and first branchial {a.br.a. 1) arteries. Next 

 behind this comes a paired vessel which arises from the dorsal 

 aspect of the aorta and supplies arteries (a.br.a. 3, a.br.a. 4) to the 

 third and fourth gills. Finally the afferent artery (a.br.a. 2) to 

 the second gill, which is the only one to come directly from the 

 aorta, is in point of origin the most posterior of all. This is 

 due, of course, to the displacement headwards of the common 

 trunk of supply to the third and fourth gills. 



The afferent hyoidean (fig. 4,a.hy.) and first branchial (a.br.a.l) 

 arteries immediately pass ventral to the tendon of the M. sterno- 

 hyoideus (m.sthy.), while all the other afferent arteries are dorsal 

 to it. The former, running close beneath the 5kin, follows the 

 hyoid arch for a considerable distance. Anteriorly it. lies just 

 medial to and parallel with the A. hyoidea (a.md.), the vessels 

 and the cartilage suggesting the arrangement of parts in a gill. 

 Further back it does not follow the arch so c'osely and runs 

 more nearly parallel with the median line. This vessel seems 

 clearly to correspond to the afferent hyoidean artery of other 

 ganoids and selachians. With teleosts, although sometimes 

 present in young (e.g., Ameiurus, Salmo), it appears typically 

 to be absent in the adult. In Polyodon I have frequently been 

 unable to find it, so its absence may here be a more or less com- 

 mon anomaly — an anomaly which might perhaps be expected 

 on physiological grounds inasmuch as this artery can distribute 

 only venous blood to the tissues. 



The first afferent branchial artery, after having passed under 

 the tendon of the sternohyoideus, turns obliquely outward and 

 backward to enter its gill along the posterio-ventral border of 

 the m. obliquus ventralis I (m.obv.l). The second enters its 

 gill in exactly the same way except that it goes dorsal instead of 

 ventral to the tendon (fig. 4). 



The trunk (a. an.) on either side which supplies the third and 

 fourth gills runs back near the median line and dorsal to the 



