MUSEUM OF COMPARATIVE ZOOLOGY. 203 



terminal branches (e. g. palato-nasal) to the anterior end of the body, 

 and there is no distinct bifurcation, the vessel, on the other hand, being 

 gradually reduced by the numerous lateral branches given off after its 

 passage through the circulus cephalicus. The aorta in Myxine is 

 a relatively large vessel until it unites with, or, better, receives the 

 two converging branches of the circulus, when it suddenly contracts, 

 and from this point on is clearly much reduced. This portion of the 

 canal still serves to keep open a direct passageway from the heart to 

 the head, inasmuch as the arterial blood collected from the gills is not 

 only forced upwards and backwards, but also forwards, into the continua- 

 tion of the aorta, or A. vertebralis impar, and into the carotid arteries. 



I think, from the evidence gathered in the foregoing paragraphs, that 

 we are now in position to say definitely that between Amphioxus and 

 the Myxinoids on the one hand, and Chlamydoselachus as a representa- 

 tive of the Elasrnobranchii on the other, it is easy to establish a homol- 

 ogy of parts surprising in its completeness. The entire dorsal aorta 

 exists in Chlamydoselachus, imbedded in part, it is true, in the basis 

 cranii, while in Myxine it lies in the connective tissue underneath the 

 latter. The long ventral aorta in these two forms has been much short- 

 ened, but still in showing traces of its reduction claims a descent from 

 an Amphioxus-like type. Of course we should not expect to find the 

 ventral aorta persisting after the gills in front of it had ceased to be 

 functional, and it might easily shorten before such reduction of the 

 branchial apparatus had taken place, provided means were at hand to 

 enable it to perform its function of pouring blood into the gills. The 

 dorsal aorta, on the other hand, being a distributing trunk in a large 

 sense, would be looked for so long as its territory existed and was 

 not entirely supplied by new vessels ; and as we know that its territory 

 persists in all vertebrates, and greatly increases in extent among the 

 higher forms, the latter alternative is the only one we need consider 

 further. We find, on examination, that the recession of the heart is 

 accompanied by the usurpation of the precardiac aortic territory by 

 some of its lateral branches or their smaller offspring. While it is true 

 in general that a reduction of the ventral aorta is followed by a reduc- 

 tion of the dorsal vessel, it is also true that the latter process takes 

 place much more slowly, and for the reasons given above. The only 

 indication of a persistence of the dorsal aorta in groups above the 

 fishes, of which I have been able to find reliable account, is given by 

 Goette. 11 



11 Goette, Alex. Entwickelungsgesehichte der Unke. Leipzig, 1875. 



