The pseudobranchial circulation in Amia calva, 109 



rectly coatinuous, at its anterior end, with the hind end of the pseudo- 

 branchial chamber, while at its posterior end it opens into the anterior 

 end of the first branchial cleft. In each of the three specimens 

 examined, in serial sections, at this stage of development this channel 

 had much more the appearance of an anterior prolongation of the 

 first branchial cleft, than that of a posterior continuation of the pseudo- 

 branchial cleft or chamber. In one of the three specimens, cut in 

 horizontal sections, this appearance was especially marked ; the channel, 

 in these sections, running at first mesially and backward, around the 

 anterior end of the adductor hyomandibularis muscle, nearly at right 

 angles to the general direction of the pseudobranchial chamber, and 

 then curving gradually backward and laterally into the first branchial 

 cleft. In a second specimen, cut in transverse sections, and of which 

 a model was made, this change of direction in the two parts of the 

 cleft was not nearly so marked. When examined, unsectioned, under 

 a dissecting microscope, the narrow, slit-like part of the pseudobranchial 

 opening is not easily recognised. The posterior part of the opening is, 

 however, large, and forms a deep pocket leading forward from the 

 anterior wall of the first branchial cleft, near its anterior end, and 

 from the adjoining dorsal surface of the mouth cavity. 



The anterior part of the oral opening of the pseudobranchial cleft 

 is thus, in 12 mm larvae of Amia, a relatively large, V-shaped groove 

 on the dorsal surface of the mouth cavity, opening, by a slit-like 

 aperture along its dorsal edge, directly into the pseudobranchial 

 chamber. This V-shaped groove begins at the hind end of the pseudo- 

 branch, and extends forward somewhat beyond the anterior end of 

 the slit-like opening that leads into the pseudobranchial chamber. At 

 the hind end of the pseudobranch the groove becomes abruptly en- 

 larged and deepened, and, as a wide and deep, rounded groove, forms 

 the posterior portion of the opening of the cleft, opening not only 

 onto the dorsal surface of the mouth cavity, but also, posteriorly, into 

 the anterior end of the first branchial cleft. In the adult, as shown 

 in one of my earlier works (1, fig. 60), the slit-like part of the 

 opening has entirely disappeared, and the pseudobranchial cleft opens 

 on the anterior wall of the first branchial cleft, near its anterior end, 

 by a relatively small, round, or oval opening which is not seen unless 

 the parts are here pressed somewhat apart. From this opening a 

 short canal leads forward into the pseudobranchial chamber. The 

 slit-like part of the opening found in 12 mm larvae thus evidently 

 becomes entirely closed, as the animal develops, by the coalescence 



