OCR Text |
Show 1899.] BLOOD-VESSELS OF TELEOSTEAN FISHES. 953 except that the subclavian arteries arise farther from the median line, just as they do in Molva. In Lota, as described and figured by Hyrtl (7. pl. i. fig. 2), there is a tendency for the last two efferent branchial vessels to unite before opening into the circulus. The fact that the coeliaco-mesenteric artery arises from the circulus is shown in Hyrtl's figure, and is quoted by Stannius (22. p. 103, footnote 3) and by Owen (19. p. 490). In Blennius (fig. 35) the circulus cephalicus is narrow in front, the coeliaco-mesenteric artery arises from the aorta immediately behind the circulus, and the subclavian arteries are just behind this. In Orthagoriscus (fig. 36) the aorta is unsymmetrical, running to the right side of the vertebra?; the coeliaco-mesenteric artery is formed by the union of a pair of vessels arising from the common trunks formed on each side by the confluence of the third and fourth efferent branchial vessels. The mesial ends of all the efferent vessels are closely approximated. The gills in Orthagoriscus are remarkably prolonged in a backward direction. The efferent branchial vessel of each emerges from near the middle of the full extent of the gill, aud not, as is more usual, from the upper end. It is formed by the union of one vessel coming from the lower part or gill proper, with another from the dorso-posterior prolongation, in a manner already made clear by Alessandrini (2) and Milne- Edwards (15. p. 335, footnote 2). Judging from Alessandrini's description, the peculiar mode of formation of the coeliaco-mesenteric artery in 0. truncatus does not occur in the species examined by him (0. mola). In Tetrodon (fig. 37) there are only three gills on each side, and three efferent branchial vessels, the fourth of the normal series being absent. The reason for putting the genus in the subgroup D c, characterized by the union of the third and fourth efferent vessels into a common trunk, is to be found in the relation of the coeliaco-mesenteric artery to the third efferent vessel of the right side. The association is such that, were the fourth vessel present, it could not reach the circulus between the third vessel and the coeliaco-mesenteric. Both the right and left subclavian arteries arise from the right side of the circulus, and the latter is nearly circular iu shape. CONCLUSIONS. It will be seen from the foregoing observations that very considerable diversity in the arrangement of tbe efferent branchial blood-vessels is to be met with in the Teleostean fishes. The type of vascular arrangement is constant for different species of the same wenus, and does not vary to any considerable extent in different o-enera of the same family. If, as in the Siluroid fishes, some widely divergent types are included in the same family, there are to be" found intermediate types which act as connecting-links between these extremes. With regard, however, to families which, in the at present accepted taxonomy of the group, are brought into close relationship, |