OCR Text |
Show 1899.] BLOOD-VESSELS OF TELEOSTEAN FISHES. 943 involving the second, and later the third and fourth efferent branchial vessels ; and secondly, by the progressive suppression in length of the median aorta, bringing about an approximation of the dorsal or proximal ends of the last three efferent branchial vessels on each side. Owing to the fact that the one variety of specialization may occur quite independently of the other, or in conjunction with it, it becomes very difficult in some cases to compare the ultimate degree of specialization attained, since there is no evidence to show whether the inclusion of the efferent vessels into the circulus or the approximation of the efferent vessels by the suppression of the aorta is the more important. In the Salmon (fig. 7), for instance, the second efferent vessel opens into the circulus cephalicus-an indication of specialization ; but a length of aorta persists between the circulus cephalicus and the third efferent vessel-a primitive character. In Balistes (fig. 5) the second vessel is free from the circulus cephalicus, and yet there is obvious specialization in the complete suppression of the aorta in the branchial region, resulting in the second, third, and fourth vessels opening close together, immediately behind the circulus. W h o shall say whether, in the disposition of the efferent branchial vessels, the Salmon or the File-fish is the more primitive? Having recourse to the other anatomical features of these two forms, one would conclude that, the Salmon being in general structure the more primitive, the abbreviation of the aorta is as a mode of specialization more important than the backward extension of the circulus to include the second efferent branchial vessels. The conclusion is further justified by tbe fact that Albula (fig. 11), which is undoubtedly allied to Megalops (fig. 4) and Chirocentrus, differs from these genera in this latter respect. This hypothesis, however, opens up the further question as to how far a backward extension of the circulus cephalicus is due to the longitudinal splitting of a part of the median aorta. Has, for instance, the condition found in Megalops and Chirocentrus, in which the circulus cephalicus extends back to the second branchial vessels, been brought about by the longitudinal division of a median vessel such as exists in Clupea (fig. 2) and Engraulis (fig. 1) between the first and second efferent branchial vessels? The suggestion has much to recommend it; more especially as the suppression of the median aorta cannot have operated here, or the two anterior carotids would be arising close together at the bottom of the fork of the first efferent branchials. Another line of specialization, independent of the two former, can be traced in the confluence of the third and fourth efferent vessels. Having assumed that the separation of two consecutive efferent branchial vessels by a portion of the median aorta is a primitive feature, it follows that the separate entry into the aorta of the third and fourth vessels in Engraulis (fig. 1) is an indication of less specialization than the debouching of the two vessels together, as in Clupea (fig. 2); and further, that this latter 61* |