24 General Pwrt. 



portion of the canal is modified to form a gizzard for the 

 trituration oi food. In some animals^ e.g., the higher Crustacea, the 

 hind part of the oesophagus widens out, becomes muscular, and is 

 furnished within with hard denticles ; in other cases, as in Birds, 

 it is the posterior region of the stomach. 



Very frequently diverticula, blind sacs of various kinds, 

 are found at different parts of the alimentary canal. Their function 

 varies : those belonging to the fore-gut serve only as provisional 

 reservoirs for the food which has just been taken in (crop in Birds 

 and Insects) ; those of other regions usually serve to enlarge 

 the canal, or to increase the digestive and absorptive surface 

 (diverticula of Leeches, cffica of Mammalia), but this may be effected 

 in other ways — by the elongation of the intestine, by delicate 

 foldings of the inte.stinal wall, or by some similar contrivance 

 ( Vertebrata) . 



The digestive fluids are secreted partly by the epithelium of the 

 digestive tract; partly by minute glands imbedded in the wall, 

 especially of the stomach and intestine; and partly also by larger 

 glands, lying outside the alimentary canal, and communicating with it 

 only by means of their ducts. In many lower animals the glandular 

 activity is associated with the epithelium of the digestive tract 

 alone, whilst among more highly organised forms, the Vertebrata, for 

 example, it may be essentially or exclusively possessed by special 

 glands. The larger glands lying without the canal are variously 

 named, according to the region into which their ducts open ; those for 

 instance, which open into i the buccal cavity, and which are often of 

 little direct use in digestion, serving only to moisten the food and to 

 facilitate its onward course, are called salivary glands; whilst 

 those which open into the intestine are usually termed liver. The 

 secretions of these glands difier in different animals ; the efiect of the 

 " liver " secretion for example, is by no means always the same. 



As to its minute structure : in its simplest form, the alimentary 

 canal consists only of a layer of epithelium, but connective tissue and 

 muscle may come to surround this, so that in a more highly 

 specialised gut, the wall is composed of several layers : within, a 

 layer of epithelium coated by a connective tissue sheath, often 

 including numerous little glands, the two being closely adherent, and 

 constituting the mucous membrane ; outside this there is a sheath of 

 muscle cells (the muscular coat) whose contraction-s are of great 

 importance for the passage of nutriment through the canal. 



In only a few Metazoa is the alimentary canal wanting. In such 

 cases, either there is a functional mouth to allow of the entrance of 

 food into the soft tissues of the body, and these perform the digestion : 

 or, there is no mouth, and the food passes through the skin of the 

 animal by endosmosis. This, however, only occurs in parasitic 

 animals, especially in such as live in the gut of other animals, where 



