^3 



MORPHOLOGY OF THE ORGAXS OF VERTEBRATES. 



Fig. 41. Ichthyosauran 

 coprolites, one in section. 

 The spiral character b taken 

 as evidence of the presence 

 of a spiral valve in these 

 reptiles. After Leunis. 



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

 cascal 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- 

 line caecum, the remains of the yolk stalk of 

 development, by which, in the earlier stages, 

 the intestine was connected with the volk. In 

 these higher forms increase of intestinal sur- 

 face is brought about in part b}' 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 

 externall)-, but from the 

 amphibia on it acquires 

 greater individualit}-. 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 



1 The marks on certain reptilian coproUtes indicate that some e.xtinct reptiles may have 

 had a spiral valve (Fig. 41). 



Fig. 42. Stom- 

 ach and pyloric caeca 

 of Salmo, after 

 Rathke. 7, intes- 

 tine; P, pyloric 

 caeca; S, stomach. 



Fig. 43. Part of small intestine of man, 

 showing the valvulae conniventes, from 



Martin. 



