THE HEART AND ARTERIES OF POLYODON 425 



corrected and elaborated the original account and extended their 

 observations to one of the ganoids, Amia. Parker ('00), Sil- 

 vester ('04) and others have studied these vessels in teleosts. 

 In Polyodon I find a considerable latitude of variation in minor 

 points even on opposite sides of the same fish, but no asymmetry 

 of constant occurrence, such as characterizes some of the tele- 

 osts, has been noticed. In figure 4 there is a slightly schematic 

 representation of the whole system in a specimen where the 

 parts attained a very full development. 



The recurrent artery from the first gill (a.md.), generally 

 designated as A. hyoidea or afferent pseudobranchial artery, 

 is apparently constant in occurrence and position. It does not 

 anastomose with the other hypobranchial arteries and in this 

 region gives off only very small branches or none whatever. 

 From its origin (fig. 7) it crosses the m. obliquus ventralis I diagon- 

 ally and loops around its tendon, passing first dorsal, then medial 

 and finally ventral to it as shown in figure 4. On rounding the 

 tendon it gains the posterio-medial aspect on the hypohyal 

 where, however, there is neither foramen nor groove. Allis ('11) 

 finds branches here to the local musculature. It crosses the 

 lateral aspect of the ceratohyal (c.ch.) in a diagonal furrow (fig. 

 16) which traverses nearly the whole length of the proximal 

 cartilaginous portion of that element. Turning dorsally it 

 passes in front of the interhyal to the lower edge of the symplec- 

 tic where it again enters a shallow groove in the cartilage. Here 

 it frequently gives rise to a vessel (fig. 16) that runs back under 

 the end of the hyomandibular. AUis (I.e., p. 260) is convinced 

 that this branch is homologous with similar vessels in Amia 

 and Salmo and that it represents in all these the ''remnant of a 

 commissure that in younger larvae undoubtedly connected the 

 hyoidean and mandibular aortic arches." It is absent in many 

 of the adult Polyodons studied. Leaving the dorsal aspect of 

 the symplectic the afferent pseudobranchial artery runs forward 

 in a course ventral to the protractor hyOmandibularis muscle, 

 but separated from the oral mucosa by a thick layer of fatty 

 tissue. Anteriorly it makes an >S-shaped bend (fig. 15) and 

 enters the pseudobranch, where it usually breaks up into several 



