1H90.] ANATOMY OF THi: AtJSTRALIAN TORPEDO. 6/3 



as a series of insignificant investments for the blood-vessels of the 

 former ', 



On turning to the Amphibia, we find that in the Anura the 

 mesentery is, like (hat of the Amiiiota, continuous, except for an 

 occasional feeble splitting and overgrowth in relation to the 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 {Ichthyophis, 

 Siphonops, Siren, Proteus, Amphiuma, Menopoma), or widely inter- 

 rupted (Salamandra', Siredon, Menohranchus), in a manner such as 

 is never realized in any known Anuian. 



The presence of a continuous mesentery can only mark the reten- 

 tion of a lowly character ; wherefore it follows that the Batoid 

 Hypnos subnigi-um, although admittedly one of the most specialized 

 living members of the oi'der Plagiostomi, retains at least one charac- 

 ter more lowly than that of all its alhes. 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 Gymnopliiona and Anura among 

 Amphibians. 



Finally, as to the rationale of the process of absorption of the 

 mesentery among the Ichthyopsida 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 hgament] ' of the 

 liver) as the broad, or falciform ligament. 



The relationships of the dorsal mesentery of the cartilaginous 

 fishes to the dorsal intestinal vein^ 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 PefromyzontidcB, there is a striking 



'^ 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 On examination of a numerous series of individuals I find this interruption 

 to be variable, and at times unefFected. 



5 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. 023) " that the falciform ligament is not a remnant of a primitive ventral 

 mesentery." Beddard's discovery would appear to me fatal to this considera- 

 tion, and it calls for a reinvestigation of the matter. 



« Cf. T. J. Parker, PhU. Trans, vol. clxsvii. (pt. ii. 1886), p. 707. 



