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Show 1890.] ANATOMY OF THE AUSTRALIAN TORPEDO. 671 (ap. of figs. 2 & 3) and that portion of the gut which lies behind it, as also to carry the post-superior (so-called inferior) mesenteric artery (a.m.", fig."2). These folds have been termed "mesogaster" and "rnesorectum"1. The first named extends backwards to the posterior limit of the so-called superior mesenteric artery (a.m.1) ; it ensheaths this vessel and its cceliac ally, and, while frequently continuous as a simple sheet, it is more generally fenestrated and broken up". Inasmuch, however, as it suspends the duodenal segment of the gut together with the stomach and pancreas, the term mesogaster would be well withdrawn. I have elsewhere attempted to show3 that that portion of the Plagiostome's gut which is suspended by mesentery posteriorly, represents the entire large intestine of the higher Vertebrata ; whether I am right or not, the term " rnesorectum " might be preferably restricted to that mesentery of those higher Vertebrata possessed of a recognizable rectum, and withdrawn here in preference to a more general one. I would propose to substitute for " mesogaster" the term ante-mesorceum, and for " rnesorectum " that of post-nnesorceum. The foregoing description applies to both the Plagiostomes and Chimseroids, allowance being made for the absence of the processus digitiformis in the latter; in them, and in some Selachii, the fenestration of the ante-mesoraeum is so complete that the superior mesenteric arteries, be there one or more present, are set free within the folds of cord-like sheaths, aud the assumption of this condition is associated with the origin of the arteries named at a point remote from the cceliac axis. From the known facts of development of the dorsal mesentery, the fenestrated condition of the same may, with tolerable certainty, be regarded as due to absorption; and the description of it as "interrupted by one or more large fenestrae " i is as satisfactory as could be wished. In Hypnos the conditions are otherwise, for this fish is, among the Chondrichthyes, the sole known possessor of a continuous mesentery. The fact that in it the spleen and processus digitiformis are not seen on opening the body-cavity from the ventral aspect, has been already alluded to. It is due to these organs being hidden beneath the continuous mesentery named. More than this however! for, on turning the alimentary viscera to the left side as indicated in fig. 2, the mesentery in question is seen to be perforated by two large round holes. The anterior of these lies immediately behind the anterior (superior) mesenteric artery (a.m.1), in juxtaposition to the head of the small intestine, and gives passage to the spleen (s.); the posterior occurs lineally below the posterior (inferior) mesenteric artery (a.m."), and similarly gives passage to the appendix digitiformis (ap.). There here arises the question whether the spleen and digitiform process, which alike perforate the mesentery, 1 Parker, T. J., ' Zootomy,' p. 47. The descriptions of Haswell for Trygon and Urolophus (Proc. Linn. Soc. N. S. W . vol. iii. (ser. 2) p. 1716) are more nearly correct. 2 Cf. Stannius,' Handbuch d. Anat. d. Wirbelth.' p. 193. 3 Journ. Linn. Soc, Zool. vol. xxiii. pp. 393 et seqq. (1890). * Marshall & Hurt, Junior Course in Practical Zoology, ed. 2, 1888, p. 218. 45* |