THE HEART AND ARTERIES OF POLYODON 



419 



its individuality, the ventral articulation is displaced forward 

 and upward to the level of a dorsal articulation, and the original 

 dorsal (if it be such) has become posterior in position and is 

 nearly obliterated. In this readjustment the artery {a.br.a. 4-) 

 has retained its original relative position and is in consequence 

 brought up to the floor of the ijiouth (fig. Q, A). 



c. CO. 3 



a. br. a. 4 



a. br. a. 4 



c. cer. 4- 



m. obv. 4- 



\J^ 



/ 



B 



Fig. 6 A, section through a portion of the hypobranchial region of Polyodon 

 (74 mm. specimen); B, a similar section of Amia at the same level (Harvard 

 Embryological Collection, No. 273, sections 270-285). 



THE BRANCHIAL CIRCULATION 



On entering their respective gills the afferent vessels (a.br.a) run 

 a considerable distance without giving rise to any filamentar 

 arteries. At a point well towards the middle of the cerato- 

 branchial region each gives off a recurrent vessel {a.rc, fig. 7) 

 which bears all the filamentar arteries medial to this points 

 Dorsally all the afferent vessels except the fourth bifurcate near 

 the end of the gill, sending a division to the two hemibranchs as 

 they diverge around the insertion of the m. levator arcus bran- 

 chialis (a.bi.). Allis (loc. cit., p. 259) discusses these vessels 

 briefly, but does not mention their terminal bifurcation. He 

 finds the recurrent branches in larvae (130 mm. to 170 mm.), 

 but in view of drawings in his possession thinks they may be 



