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Show 1890.1 ANATOMY OF THE AUSTRALIAN TORPEDO. 673 as a series of insignificant investments for the blood-vessels of the former \ O n turning to the Amphibia, we find that in the Anura the mesentery is, like that of the Amniota, continuous, except for an occasional feeble splitting and overgrowth in relation to tbe gathering up of the blood-vessels within the folds of the gastro-duodenal omentum (ex. Ceratophrys). In the Urodeles, however, the mesentery is either continuous and unabsorbed (Lchthyophis, Siphonops, Siren, Proteus, Amphiuma, Menopoma), or widely interrupted (Salamandra2, Siredon, Menobranchus), in a manner such as is never realized in any known Anuran. The presence of a continuous mesentery can only mark the retention of a lowly character ; wherefore it follows that the Batoid Hypnos subnigrum, although admittedly one of the most specialized living members of the order Plagiostomi, retains at least one character more lowly than that of all its allies. It is interesting to note the parallelism to this which is seen in the other orders of Vertebrata cited, the dorsal mesentery being complete in the specialized Hags among Marsipobranchs, and in the Gymnophiona and Anura among Amphibians. Finally, as to the rationale of the process of absorption of the mesentery among (he lchthyopsida in general. The Dipnoi and Amphibia are well known to possess a median ventral mesentery, which, in the last-named order, lodges the median epigastric (anterior abdominal) vein. This mesentery, like its fellow on the dorsal side of the gut, is well known to be subject to absorption ; and if a Frog and a Salamander be compared, it will be readily seen that in its most completely absorbed state it forms but a cover for the vein named. In the Amniota it becomes still more abbreviated, and finally persists in relation to the median epigastric vein (or its homo-logue the umbilical vein [afterwards the round ligament]3 of the liver) as the broad, or falciform ligament. The relationships of the dorsal mesentery of the cartilaginous fishes to the dorsal intestinal vein4 repeat those of the ventral mesentery of the Amphibia and Amniota to the median epigastric vein. Again: between the relationship of the first named to the intestinal arteries, in those Plagiostomes in which it is most completely absorbed and in the Petromyzontidce, there is a striking 1 I think it not unlikely that its disappearance in these fishes has to do with the immense development of the genital glands, they having apparently fused in the middle line. 2 O n examination of a numerous series of individuals I find this interruption to be variable, and at times uneffected. 3 Beddard has briefly described (P. Z. S. 1884, p. 553) a median epigastric vein in the adult Echidna. It is most desirable that the relationships of this vessel should be more fully worked out. I cannot reconcile with' this the belief (Balfour, Comp. Embryology, vol. ii. p. 623) " that the falciform ligament is not a remnant of a primitive ventral mesentery." Beddard's discovery would appear to m e fatal to this consideration, and it calls for a reinvestigation of the matter. * Cf. T. J. Parker, Phil. Trans, vol. clxxvii. (pt. ii. 1886), p. 707. |