94 GENERAL PRINCIPLES OF ZOOLOGY 



which have only a single opening, functioning both as mouth and anus 

 (fig. 58). Such an animal has necessarily at least two epithelial layers, 

 one of which lines the digestive tract, the other covers the surface of the 

 body. These two fundamental cell-layers are called entoderm and ecto- 

 derm. In many ccelenterates they are the only layers of the body. In 

 most animals they are separated by intermediate tissues, called collec- 

 tively mesoderm. The higher the animal, the more differentiated is the 

 mesodermal layer. The primitive digestive cavity lined by entoderm is 

 called the archenteron. In the case of medusre and polyps (fig. 58) it 

 forms the entire digestive tract, but in most animals this is not sufficient 

 for the needs of digestion and the alimentary tract is increased by invag- 

 inations of parts of the surface (ectoderm) of the body. 



Stomodaeum and Proctodaeum. Even in many ccelenterates and 

 lower worms an imagination arises at the anterior end of the digestive 

 tract, forming the ectodermal fore-gut or stomodffum (fig. 59). From the 

 higher worms onwards, it is accompanied by a second imagination at the 

 hinder end, the ectodermal hind-gut, or proctodceum (fig. 60) ; embryolog- 

 ically, this is formed as a blind sac whose closed end unites with the like- 

 wise closed posterior part of the archenteron (now called also mesenteron 

 or mid-gut) until the separating wall disappears, whereupon mid- and 

 end-gut communicate with each other, and the digestive tract becomes 

 a canal extending through the entire body. 



The part which the archenteron takes, in comparison with the ectodermal 

 proctodteum and stomodaeum in making up the completed digestive tract, is very 

 different in the various groups. On one side the Crustacea, on the other side 

 the vertebrates, offer the strongest 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. 



Divisions and Appendages of the Digestive Tract. The width 

 of the lumen varies in the course of the alimentary canal and renders 

 possible the recognition of different divisions, which, so far as possible, 

 have been provided with uniform names. Fig. 61, 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 buccal 

 cavity, and a posterior one, the pharynx. The narrow tube leading from 

 this is the oesophagus (a) ; here and there it may widen, or bear a pouchlike 

 evagination, the crop or in glumes (b), 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 



