276 FISHES CHAP, IX 
Fishes! 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, Polyodon), 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.” The organ is generally 
present in Elasmobranchs (Fig. 155, ret.g/), 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,* but 
communicates directly with the cloaca (Fig. 155, A, cl.c). The 
“rectal” gland is perhaps homologous with the intestinal caecum 
which is to be found in some Teleosts (e.¢. Box vulgaris), and 
possibly also with the “caecum” (caecum coli), and its vermiform 
appendix in the higher Vertebrata.®° The caecum cloacae, on the 
contrary, 1s 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.® 
1 Wiedersheim, op. cif, p. 556. 
2 Howes, op. cit. p. 393. 
3 Giinther, Challenger Reports, ‘‘ Zool.” xxii. 1887, p. 3; Garman, Bull. Aus. 
Comp. Zool. Camb. Mass. xii. 1885, p. 20. 
+ Giinther, op. cit. p. 545 ; Newton Parker, op. cit. p. 137. 
° Howes, op. cit. p. 398 et seq. 
8 Graham Kerr, P.Z.S., 190], ii. p. 484. 
