672 PROF. G. B. HOWES ON THE VISCERAL [DeC. 2, 



may have actually caused the absorption of its substance ; or whether 

 the latter may have not been induced by other means, the glands 

 named having merely accommodated themselves to the exigencies of 

 the case. The spleen had been unfortunately removed, to a large 

 extent, in my specimen before it reached my hands (c/". Plate LVII. 

 fig. 3, s.) ; sufficient, hovyever, remains to show the presence of a 

 couple of well-marked furrows. One of these (/.p-) indicates the 

 point of apposition with the head of the pyloric gastric sac (py.) ; 

 the other (/■m.) that of strangulation or embrace by the mesentery. 

 That portion of the spleen which lay to the right of the latter (= 

 that marked s. in figs. 2 & 3) had grown out into a veritable heruia. 

 In view of the very variable extent to which this organ may force 

 its way between the folds of the mesentery among Plagiostomi in 

 general, I am inclined to adopt the second of the two alternatives 

 postulated above, and to regard the absorption as perhaps not 

 primarily due to this hernia-like extension of the glandular struc- 

 tures named. 



The spleen of the Batoidei is remarkable among that of all verte- 

 brates for its enormous development. It lies in the bay formed 

 between the cardiac and pyloric sacs of the stomach, and projects 

 freely to the right side. It either extends under cover of the stomach 

 and intestine, giving rise to a solid mass which lies immediately 

 beneath the backbone, and not unfrequently fills the interspace 

 between the genital glands ; or it embraces the left side of the valved 

 segment of the gut (ex. Rhinobatus and Trygo7iorhind), in a manner 

 somewhat resembling that in which the embryonic supra-renal 

 body of mammals "caps" its corresponding kidney. It will be 

 observed that in Hypnos the spleen (s., fig. 2) passes behind the 

 main trunk of the (anterior) superior mesenteric artery {a.m.'). In 

 many Plagiostomes, it shows a marked tendency to extend either in 

 front of the same or between its branches ; consequently, while the 

 facts seen in the absorption of the mesentery of Hypnos beyond 

 doubt furnish the clue to the rationale of this process as it applies 

 to the living Chondrichthyes generally, they would appear to denote 

 the initial phase in one of a possible series of variations in the 

 same. 



The absorption of the mesentery is a phenomenon which has long 

 been recognized among other vertebrated animals, and consideration 

 of the facts concerning it yields an interesting result. Rathke has 

 long ago described it' in the Turbot and Gar-Pike, Owen in 

 the Pipe-fish ", while both these observers have recorded it for the 

 Cyprinidse. 



While in the Myxinoids the mesentery is continuous, in the 

 Petroniyzontidae it is absorbed to the maximum degree — persisting, 

 as is well known, at the extreme anterior and posterior ends of that 

 portion of the gut which lies within the post-pericardiac coelom, 



^ ' Ueb. den Damkanalen unci Zeugunssoi-gane d. Fische.' Halle, 1824, 

 pp. 104-105. 



^ Comp. Anat. & Phys. vol. i. p. 424. 



Cf. also Ouvier and Valenciennes, Hist. Nat. des Poissons, vol. i. p. 507. 



