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 " raesorectum " ^ The first named extends backwards to the 

 posterior limit of the so-called superior mesenteric artery {a.m.') ; it 

 ensheaths this vessel and its cceliac ally, and, while frequently con- 

 tinuous 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 meso- 

 gaster would be well withdrawn. I have elsewhere attempted to 

 show^ that that portion of the Plagiostome's gut which is sus- 

 pended by mesentery posteriorly, represents the entire large intestine 

 of the higher Vertebrata ; whether I am right or not, the term 

 " mesorectum " might be preferably restricted to that mesentery of 

 those higher Vertebrata possessed of a recognizable rectum, and with- 

 drawn here in preference to a more general one. I would propose 

 to substitute for " mesogaster " the term ante-mesorceum, and for 

 " mesorectum " that of posi-mesorcBum. 



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 fenes- 

 tration of the ante-mesorseum is so complete that the superior 

 mesenteric arteries, be there one or more present, are set free 

 within the folds of cord-like sheaths, and 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 " in- 

 terrupted by one or more large fenestrse " * 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 con- 

 tinuous 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.'), in juxta- 

 position 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, 



^ Parker, T. J., ' Zootomy,' p. 47. The descriptions of Haswell for Trygon 

 and Urolophus (Pvoc. Linn. Sec. 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. 



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