INTESTIITAL CANAL OF THE ICHTHTOPSIDA. 403 



the caecum coli and appendix digitiformia of certain other 

 Vertebrata (of. ante, p. 396). 



An ileo-colic valve is not differentiated. 



It may be urged, against my belief in an homology between 

 the above-described caecum of the Teleostei and the caecum coli 

 of the higher Vertebrata, that the intestinal {ileo-colic) valve of 

 the former appears to lie within the area of the so-called spiral 

 valve of the Clupeoid Chirocentrus * and of the annulations of 

 the intestinal mucous membrane in the Salmonidse and other 

 Teleostei {cf. Eathke, 29. pp. 62 et seq.). In Cliirocentrus the 

 prevalvular gut is short, and the intestine passes to the exterior 

 without convolution. In the Salmonidae the prevalvular gut is 

 elongated and bent upon itself. Huxley, writing on this sub- 

 ject, says (/. c. p. 138) : — " I am inclined to believe that the 

 circular valve which separates the colon from the rectum in the 

 Smelt is merely a last remainder of the spiral valve " (of the 

 Ganoids and Chirocentrus). This valve occupies in the Smelt 

 precisely the position of that which I have been led to compare 

 {ante, p. 401), in other Teleostei alluded to, with the ileo-colic 

 valve of the higher Yertebrata. Assuming, for the moment, tha't 

 the annulated segment of the gut in the Trout and Salmon and 

 the post-valvular portion in the Smelt are homologous, and repre- 

 sentative of the large intestine of other Teleostei, examination of 

 these in the order named might, at first sight, appear to show 

 evidence of a gradual diminution in length in passing from the 

 former to the latter. Eathke, six-and-thirty years ago, drew 

 attention {I. c.) to the fact that in the Salmonidse the annular folds 

 alluded to are susceptible to great variation and difference with 

 age. He, nevertheless, regarded them as restricted to the 

 rectum. If the lining membrane of the Smelt's intestine be care- 

 fully examined it will be found to be produced into a very 

 obvious series of crenulated annular folds, throughout the area 

 occupied by the more definite series of the Salmon and Trout. 

 The valve to which Huxley alludes, and which I regard as the 

 probable homologue of the ileo-colic valve, lies within an inter- 

 rupted area in this series — in the Salmon and Trout it is absent. 



* Cuvier and Valenciennes, 'Hist. nat. des Poissons,' t. xix. p. 151, pi. 565. 

 Cf. Huxley, P. Z. S. 1883, p. 138. 



