lOG GENERAL PlilNGIPLES OF ZOOLOGY. 



contrast; the Crustacea have a very short mid-gut and consequently 

 a long extent of fore- and hind-gut formed from the ectoderm; in 

 vertebrates, on the contrary, the ectodermal portions are extremely 

 short. 



The width of the lumen varies in the course of the alimentary 

 canal and renders possible the distinction of different divisions, 

 which, so far as possible, have been j^rovided with uniform names. 

 Fig. 57, drawn from a domestic fowl, illustrates the usual terms. 

 The mouth-opening leads into a wider cavity, which is usually 

 divided into an anterior division, the liuccal rMvity, and a posterior 

 one, the pharynx. The narrow tube leading from this is the 

 msophagios (a) ; here and there it may widen, or bear a pouchlike 

 evagination, the crop or ingluvies [h), for the temporary reception 

 of food. From the oesophagus the food passes into a considerable 

 enlargement, the stomach. Birds, like many other animals, have 

 a double stomach, a thin-walled portion rich in glands, and a 

 second part, the walls of which are remarkable for the thick 

 masses of muscle; the former is the (jlundular stomach (c), the 

 latter is the grinding stomach or gizzard (c/), serving for comminu- 

 tion of the food. Behind the stomach the digestive tube narrows 

 into the small intestine (h), the hinder widened part of which is 

 the large intestine (l), terminating in tlie anus. The limit of the 

 small and large intestine is usually marked by blind pouches, the 

 ccera (k). Connected with the anal gut also are the outlets of the 

 kidneys (m) and of the sexual ap23aratus {«); hence the terminal 

 portion, serving as the outlet for the urine and faces, and also for 

 the sexual products, is called the cloaca (o). 



In animals which require abundant food the area of the 

 alimentary tract is not sufficient to furnish the digestive fluids, so 

 that evaginations of the wall (glands) serve to increase this. Into 

 the mouth empty the salivary glands; into the first part of the 

 small intestine, close behind the stomach, the liver {e) and the 

 pancreas {g) (or a single glandular apparatus, whose secretion 

 combines the characters of gall and of pancreatic juice, the hepato- 

 pancreas). Finally, in the hind-gut there sometimes occur glands 

 wliicli form a fetid secretion — the aiad glands. The length of the 

 digestive tract is chiefly inlluenced by the kind of food. In many 

 groups of animals there is found a difference between herbivores 

 and carnivores, the former having a very long and consequently 

 convoluted digestive tract. That of a carnivore is about four or 

 five times the length of the body, while in an herbivorous ungulate, 

 on the otlier hand, it is t\\'enty to twentv-eii;ht times. Similar, 



