MORPHOLOGY OF ANIMALS 



are derived from the fore and hind-guts. In all the higher 

 animals the fore and mid-guts may be subdivided into 

 mouth-cavity, pharynx, oesophagus, stomach, and intes- 

 tine (Fig. 12, A and B) and in some cases these portions 

 may be further subdivided, as for example in birds where 

 the oesophagus gives rise to an enlargement, the crop, the 

 stomach is divisible into a glandular stomach and a grind- 

 ing stomach, or gizzard, and the intestine consists of two 

 portions, the small and the large intestine. Finally into a 

 portion of the hind-gut the excretory and sexual ducts as 

 well as the intestine may open, in which case this common 

 chamber is called the cloaca (Fig. 12, B, cV). 



Various portions of the fore-gut may be armed with 

 teeth, usually of a horny character among invertebrates, 

 and of calcareous material in vertebrates; salivary glands 

 open into it, and both mouth and pharynx may be pro- 

 trusible. The digestive and absorptive surfaces of the mid- 

 gut may be increased in three ways, — either ( 1 ) by an in- 

 crease in length, in which case it becomes folded or coiled, 

 or (2) by folds which project into the canal, or (3) by 

 diverticula, that is blind sacs or tubes, which open out 

 from the canal ; in many higher forms all of these methods 

 coexist in the same individual. The extent of the digestive 

 surface depends primarily upon the character of the food ; 

 if the latter is highly nutritious the digestive surfaces are 

 much smaller than where it is poor in nutrition. In car- 

 nivorous mammals, for example, the alimentary tract is 

 from four to five times the length of the body, whereas 

 in certain herbivora it may be from twenty to thirty times 

 the length of the body. 



