406 PEOF. &■ B. HOWES ON THE 



exists in the class Pisces, but is present, in a low order of the 

 same, in that which may probably prove to be its original form, 

 I fully believe. Moreover, the facts go to show that in certain 

 Teleostei {Trigla, Cyclopterus) the small csecum may possibly dis- 

 appear as the intestine increases in convolution ; and it therefore 

 becomes a matter of interest to inquire whether the first-named 

 structure may not be more generally represented in at least the 

 young of that order. 



V. In Conclusion. 

 The deductions which I have drawn are based chiefly upon 

 facts arising out of a comparison of leading blood-vessels ; and I 

 fully anticipate that it will be doubted whether these are to be 

 trusted, to the extent claimed, as guides to homology. Precedent 

 might be cited to suggest that they are not — the recent investi- 

 gations of Boas (3) and Zimmermann (34) into the aortic 

 arches, of Hochstetter (12) into the post-caval and azygos veins, 

 and of Parker (27) into the lateral epigastric veins, justify the 

 belief that they are. Objections may be here raised on the 

 ground that the great vessels just named do not undergo 

 variation such as do those of the intestinal series upon which 

 I am relying. Against this there must be set the fact (vy^hich, so 

 far as I can ascertain, has escaped notice) that the fifth aortic 

 arch, which Boas has so successfully shown (3 et op. cit.) to be 

 present between the aorta and the pulmonary artery of the 

 Urodeles, may be {Salamandra) either absent on one or both 

 sides or, when present, variable between the condition of a 

 widely open tube and that of a vestigial cord of insignificant 

 proportions. 



the male Ohimceroid is an obstacle in tlie way of a belief in its homology 

 with the processus digitiformis of the Plagiostome. 



Griinther has described (Phil. Trans. 1871, part ii. pp. 546-47) the genital 

 ducts of Ceratodus as opening, together with the ureters, into a " urinal cloaca ;" 

 and the latter would appear, at first sight, to answer to the CEecum cloacae of 

 Frotopterus. I have not been able to examine the male Ceratodus, but I think 

 it tolerably certain, on examination of the female, that Parker's " ciecum 

 cloacse " and Giinther's " urinal cloaca " will prove to be distinct in origin, and 

 that the latter will be found to represent that portion of the Elasmobranch's 

 cloaca which I herein term {ante, p. 383) the " oviducal recess " (cV", PL I. 

 fig. 1). Indeed this has been, in a sense, already anticipated by Giinther, who 

 refers to the sac on one j)age (Z. c. p. 546) as " a dorsal diverticle of the rectum," 

 and on the next (p. 547), in somewhat contradictory terms, as " the cloacal 

 dilatation," 



