2/6 FISHES CHAP, ix 



Fishes. 1 To some extent the reciprocal variation of these 

 structures supports this view, but it is also evident that there 

 are obvious objections to its unqualified acceptance. Thus, in 

 some Teleostomi (e.g. Acipenser, Polyodori}, exceptionally well- 

 developed and numerous caeca and a spiral valve are both 

 present. Amia with an almost vestigial spiral valve has no 

 trace of pyloric caeca, and in Teleosts the absence of a spiral 

 valve is associated with the complete suppression of the caeca in 

 many large and important groups. 



The Rectal Gland. The " rectal " gland, or appendix digiti- 

 formis, is a small organ of unknown function with complex 

 glandular walls, and a central duct opening dorsally into the 

 terminal portion of the intestine. 2 The organ is generally 

 present in Elasmobranchs (Fig. 153, rct.gl), in which group the 

 intestinal orifice of its duct may either be close to the termina- 

 tion of the spiral valve, or, as in Chlamydoselachus? near the 

 cloacal outlet of the gut. An apparent representative of the 

 gland, the " caecum cloacae," is also present in the Dipnoi, 4 but 

 communicates directly with the cloaca (Fig. 155, A, c/.c). The 

 " rectal " gland is perhaps homologous with the intestinal caecum 

 which is to be found in some Teleosts (e.g. Box vulgaris), and 

 possibly also with the " caecum " (caecum coli), and its vermiform 

 appendix in the higher Yertebrata. 5 The caecum cloacae, on the 

 contrary, is morphologically a urogenital sinus, formed as a 

 dilatation of the fused hinder portions of the mesonephric ducts, 

 and probably comparable with the sperm sacs of male Elasmo- 

 branchs, and also with the urinary bladder of Teleostomes. 6 



1 AViedersheim, op. cit,, p. 556. 



2 Howes, op. cit. p. 393. 



3 Giinther, Challenger Reports, "Zool." xxii. 1887, p. 3; Carman, Bull, Mus. 

 Corn.p. Zool. Carnb. Mass. xii. 1885, p. 20. 



4 Giinther, op. cit. p. 545 ; Newton Parker, op. cit. p. 137. 



5 Howes, op. cit. p. 393 et seq. 



6 Graham Kerr, P.Z.S., 1901, ii. p. 484. 



