INTESTINAL. TRACT OF MAMMALS. 



443 



Fig. l. 



Intestinal Tract of OrnitJwrhi/nrlnin nnatinvs. 



S. Cut end of duodenum next stomach. 

 R. Cut end of rectum near cloaca. 

 C. Caecum. 

 C.L. Colic loop. 

 x. Cut ends of posterior mesenteric vein. 



straight portion of the gut (that from the lower portion of "#" to R in fig. 1) corre- 

 sponds only superficial] ywith the rectal portion in Birds, in which, except in a few very 

 isolated forms, such as the Struthious Birds and the Screamers, the paired caeca mark 

 the beginning of the straight portion, whereas in the Monotremes, as in Mammals 

 generally, the homological equivalent of the straight portion of Birds extends to " C," 

 the single representative of the paired cyeca. Thus, in the morphological features 

 underlying the pattern, the; bird-like gut of the Monotremes is mammalian. The slight 

 differentiation of the colic loop is as primitive in Mammals as the absence of a specialised 



