38 MORPHOLOGY OF THE ORGANS OF VERTEBRAl^ES. 



found higher in the scale. 1 In the higher fishes it is replaced by 

 caecal tubes (pyloric appendages) developed close to the pylorus. 



The number of these 

 varies from one in cer- 

 tain ganoids to over one 

 hundred and fifty in the 

 mackerel. 



In the amphibia 

 and reptiles the mid 

 gut is nearly straight 

 in the elongate forms, 

 more convoluted in 

 the shorter types, the 



convolutions increasing in extent in the birds 

 and mammals. In the birds, at about the mid- 

 dle, the mid gut bears a blind tube, the vitel- 



FIG. 41. Ichthyosauran 

 coprolites, one in section. 

 The spiral character is taken 

 as evidence of the presence 

 of a spiral valve in these 

 reptiles. After Leunis. 



tine; P, pyloric 

 S, stomach. 



FIG. 42. Stom- 

 ach and pyloric caeca 



line caecum, the remains of the yolk stalk of of Sal mo, after 



T i i 1-1-1 i- Rathke. /, intes- 



development, by which, in the earlier stages, 

 the intestine was connected with the yolk. In 

 these higher forms increase of intestinal sur- 

 face is brought about in part by the lengthening of the intes- 

 tine, and in part by the development of numerous small folds 



(valvulae conniventes) and 

 minute finger-like projec- 

 tions (villi) resembling the 

 pile of velvet. 



The hind gut is hardly 

 distinct in fishes, as viewed 

 externally, but from the 

 amphibia on it acquires 

 greater individuality. It 

 may consist merely of a 

 straight tube, rectum, or it 



may have a terminal rectum connected with the mid gut by a 

 more or less convoluted tube, the colon. Just behind the ileo- 

 colic valve in the forms from the turtles upwards is developed a 



FIG. 43. Part of small intestine of man, 

 showing the valvulse conniventes, from 

 Martin. 



1 The marks on certain reptilian coprolites indicate that some extinct reptiles may have 

 had a spiral valve (Fig. 41). 



