392 PEOF. G. B. HOWES OIN" THE 



sentative of tlie same in the majority of living Vertebrates may 

 have arisen, by concrescence, in a manner analogous to that of the 

 allied inferior mesenteric set *. Moreover, the retention, among 

 the tailed Amphibia, of that which would appear to be the lowest 

 stage in the development of the intestinal arteries becomes the 

 more interesting and suggestive in view of the lowly condition of 

 the urinogenital organs of these animals t- 



From the foregoing I would deduce the belief (i.) that the lead- 

 ing arteries of the alimentary canal of the Yertebrata may be 

 resolved into two well-marked series — («) tliat (including the 

 cceliac, superior or anterior mesenteric, lieno-gastric, and sper- 

 matico-mesenteric arteries of authors, together with the so-called 

 inferior or posterior mesenteric of the Plagiostomes) which. I 

 would propose to term \)a.% anterior splanchnic series ; and (&) that 

 embracing the inferior mesenteric arteries as herein defined 

 {ante, p. 388), for which the term posterior splanchnic series may 

 sufl&ce : (ii.) that the two series of arteries are always separated 

 by a more or less wide interval, which increases in proportion as 

 each becomes modified by concrescence : and (iii.) that such modi- 

 fication, although common to the two series, affects the posterior 

 one in the least marked degree %• The vessels of the anterior 

 splanchnic series supply the alimentary canal and its appended 

 glands from at least the base of the oesophagus to the head 

 of the large intestine, where such can be definitely recognized ; 

 while those of the posterior series are invariably restricted to 

 the posterior portion of the large intestine (colon in part and 



* It is pertinent to remark here that examination of a series of individuals of 

 our commonest Ichthyopsida will generally reveal facts of similar significance. 

 In the Gadidce it is the rule to find the cceliac and superior mesenteric arteries 

 arising conjointly {cf. Johannes Miiller's ' Myxinoiden,' Fortsetzg. 3, pi. iii. fig. 13, 

 and Stannius's 'Anat. d. Wirbelth.,' Zweite Aufl. p. 245) ; but individuals in which 

 they arise independently daily reach our laboi-atories {cf. Parker, ' Zootomy,' 

 p. 113, fig. 32). Similarly, individuals of the common Frog occasionally present 

 themselves in which an identical condition may be seen {cf. Huxley & Martin's 

 'Practical Elem. Biology,' revised ed. 1888, p. 87). 



t Cf. Balfour, " On the origin and history of the Urinogenital Organs of 

 Vertebrates," Journ. Anat. & Phys. vol. x. p. 28 (1876), and Jungersen[20], 

 pp. 192 et seq. 



J The facts obviously suggest a likeness to the metameric symmetry of the 

 offshoots of the intestiual vessels of the segmented worms; but further specu- 

 lation under this head would be now premature. 



