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Show 1 9 0 6 .] SYSTEM OF CE 111 AIN SPECIES OF ANURA. 1 0 2 1 with the large intestine, giving off twigs on the way, to the cloacal region; but in this respect R. hexadactyla was an exception, the main vessel running freely in the mesentery beside the gut, while its subdivision formed a series of anastomoses which gave it a fenestrated structure (text-tig. 147, p. 1019). Near the distal end of this posterior vessel an anastomosis is often formed with the A. mesenterica in ferior from the aorta, but the occurrence or nonoccurrence of this seems to be quite accidental. In connection with the A . mesenterica anterior a point may be mentioned here which possibly has not escaped the notice of others, namely, the giving off of a vessel from a branch of the artery to a region of the intestine quite outside the sphere with which that branch and its vessels are concerned. Notwithstanding the approximation of different regions of the intestine that may occur owing to the folding of the mesentery, there is normally no true overlapping of fclie vessels; that is to say, their respective regions of distribution remain distinct, and even the minor vessels do not cross one another. But in certain cases a departure from this occurred. Thus in R. clamata (text-fig. 145, p. 1015) one of the vessels of the median ramus crossed under all its fellows behind it and went out of its way, so to speak, to the extreme end of the small intestine; and in a more remarkable degree, one of the vessels of the proximal ramus struck across under the whole of the median ramus to a point on the intestine behind all the vessels of the latter. Both of these points were repeated in the second specimen of R. clamata. In R. catesbiana there was no crossing of the vessels of the median ramus, but a vessel from the proximal ramus passed back under all those of the median ramus, reaching the intestine behind them ; and the same was the case in B. mauritanicus. In R. temporaria one individual had the same irregular vessel from the proximal ramus only ; another individual had two such vessels from the median ramus only. In the other species nothing of the kind occurred. There was sometimes a very close attachment between these irregular vessels and those crossed by them, but there was no union between them. In fact, while the extreme distal portions of the arteries often combine, even so as to form a continuous vessel along the wall of the gut, I observed no case of anastomosis where the vessels lie free in the mesentery, excepting the fenestrated arrangement previously mentioned in R. hexadactyla near the wall of the large intestine, and a union between two vessels of the A. gastrica sinistra close to the stomach in one specimen of R. clamata. A a. urogenitales.-These arteries have often been inaccurately described as arising from a series of unpaired stems which bifurcate right and left. As pointed out by Gaupp, such an arrangement is subject to much variation. In two of the species under consideration, namely R. hexadactyla and B. boreas (text-fig. 149, p. 1023; text-fig. 151, p. 1026), they all of them arose from unpaired stems in the aorta, which divided into right and left branches to the urogenital organs on either side. But in all other cases, traces o O ' |