THE DIGESTIVE AND RESPIRATORY SYSTEM 263 



ment of a complicated alimentary canal is shown by the com- 

 parison of various phylogenetic stages (Fig. 72). At first the 

 canal does not much exceed the body in length, and its wind- 

 ings consist of a few open curves, although in actual cases the 

 length may vary as the quality of the food, and it may thus 

 happen that in two closely allied forms considerable differ- 



FIG. 72. Comparative diagrams of the alimentary canal. 



(A) Fish. (B) Bird. (C) Mammal. 



I, pharynx; II, oesophagus; III, stomach; IV, duodenum; V, intestine; VI, cloaca; 

 g, salivary glands; r, thyreoid gland; m, thymus gland; /, bronchi, leading to the 

 lungs; h, liver; j, pancreas; k, spleen; y, pyloric diverticula (in fishes); z, cloacal 

 diverticula (in birds) ; x, intestinal diverticulum, the ccecum (in mammals) ; a, ap- 

 pendix (the narrowed free end of the latter in man). In B the stomach, III, is in 

 two parts, glandular and muscular; in C the intestine is differentiated into Va, 

 small intestine; Vb, ascending colon; Vc, transverse colon; Vd, descending colon; 

 and Ve, the rectum. 



ence may be seen in this particular. Indeed, a great contrast 

 in the length of the canal may occur in various stages of the 

 same animal, as in the frog, the tadpole of which possesses 

 a spiral, much coiled intestine, while that of the adult shows 



