Psendobranchial and Carotid Arteries iu the Gnathostome Fishes. 115 



by Fg, is not stated; but as the blood must somehow get into the 

 arteria profunda cerebrale of Raffaele's descriptions, and as the 

 vessels V^ — F5 are said by him to later become parts of the venous 

 system, it would seem as if the arterial current must necessarily 

 follow the outer line of vessels shown in Raffaele's fig-. 18. The 

 accompanying- Fig. 8 shows what would seem to necessarily be the 

 condition at this stage, but while it may correctly show exceptional, 

 or perhaps simply transient, conditions in this fish, it can not show 

 the conditions in any vertebrate in which the internal carotid re- 

 presents the primary anterior prolongation of the lateral aorta. On 

 the contrary, it would seem to show that the internal carotid is 

 developed, as claimed by Ayers (1889), from the dorsal commissures 

 of certain premandibular aortic arches; a conclusion which Raffaele 

 would probably not be willing to accept. The subject evidently 

 needs further investigation. 



The arrangement of the vessels in ganoids and teleosts can now 

 be considered, and it will be best to begin with Amia. 



In Aiuia I have already fully described, and diagrammatically 

 illustrated the pseudobranchial and related arteries (Allis, 1897 

 and 1900). but as the accompanying diagram, Fig. 10, differs slightly 

 from my earlier ones, a brief description of the vessels should be 

 given. In young larvae the common carotid and the basal portion 

 of the internal carotid of either side together form a most evident 

 and direct anterior prolongation of the corresponding lateral dorsal 

 aorta. In these young larvae, from 6 mm, to 10 mm in length, I 

 did not trace the carotid forward beyond the mandibular aortic 

 arch, and in my figures I do not show an arteria ophthalmica magna; 

 probably because it was a small and apparently unimportant vessel. 

 In older larvae, the internal carotid, in its premandibular course, 

 enters the cranial cavity, and there separates into three parts, one 

 of which is the optic artery, and the other two the anterior and 

 posterior cerebral arteries. The posterior -cerebrals of opposite sides 

 are connected by a short cross-commissure, this commissure closing 

 the circulus cephalicus anteriorly, but evidently being the homologue 

 of the anterior anastomosis shown in my figures of Mustelus and 

 llaja, and not of the posterior one. The latter commissure seems 

 wholly wanting in Amia, for no other anastomosis or commissure' 

 between vessels of opposite sides is described by me, and that I 

 did not overlook such a commissure finds confirmation in Wright's (1885) 

 statement that "no circulus cephalicus is formed by the internal carotids". 



8* 



