The pseutlobranchial circulation in Amia calva. 117 



that author says that there is, in Amia, "no trace of an afferent branch 

 from the ventral arterial trunk to the hyoidean arch". This is an 

 evident oversight on this author's part for such a branch is found in 

 all stages of the fish that I have examined, from embryos 7 mm in 

 length up to larvae of 50 mm. In the adult I have not looked for 

 it, but I do not doubt that it exists at that stage also. 



In 12 mm larvae this afferent artery to the gill cover arises, 

 with its fellow of the opposite side of the head, as a median trunk, 

 from the ventral aspect of the anterior end of the truncus arteriosus, 

 between the points where, on each side, the afferent artery of the 

 first branchial arch is given off. This median trunk separates at once 

 into two parts each of which first runs downward, forward, and 

 slightly outward, on its own side of the head, between the sterno- 

 hyoideus and branchiomandibularis muscles, lying mesial to the former 

 and lateral to the latter. It soon reaches the dorsal surface, or mesial 

 edge, of a tendinous formation that edges the mesial edge of the 

 anterior part of the hyohyoideus muscle of its own side of the head. 

 This tendinous edge of the hyohyoideus muscle is formed by the 

 fusion of the lateral part of the muscle of its own side of the head 

 with the tendinous anterior end of the mesial part of the muscle of 

 the opposite side, the tendon of this latter muscle here crossing the 

 middle line of the head. Lying in this position the artery turns for- 

 ward, and when it reaches, approximately, the transverse level of the 

 hind edge of the hypohyal, it passes downward along the mesial edge 

 of the hyohyoideus and then laterally across the ventral surface of the 

 muscle, the muscle here being almost entirely tendinous. Having 

 reached the lateral edge of this tendinous part of the muscle the 

 artery passes slightly upward and then backward and reaches the 

 dorsal surface of the muscular part of the hyohyoideus of its own 

 side of the head , lying between that muscle and the ventro-mesial 

 aspect of the ceratohyal. Continuing backward it enters the gill 

 cover, where it recedes gradually and continuously from the ventro- 

 mesial edge of the ceratohyal, lying always immediately dorsal to the 

 hyohyoideus muscle, between it and the internal epithelial lining of 

 the gill cover. Posteriorly its branches could not be satisfactorily 

 traced, but it seemed almost certain, as already stated, that certain 

 of them connected directly with the terminal branches of the posterior, 

 or opercular branch of the primary afferent pseudobranchial artery. 

 That there is also an extensively developed capillary connection 

 between the two arteries is evident. 



