94 GENERAL PRINCIPLES OF ZOOLOGY 



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

 (tig. sS). Such an animal has necessarily at least two epithelial layers, 

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

 bodv. These two fundamental cell-layers are called cnlodcrm and cclo- 

 dcnii. In many coelenterates they are the only layers of the body. In 

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

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

 mesodermal la\er. The primitive digestive cavity lined by entoderm is 

 called the arclientcron. In the case of meduss and polyps (tig. 58) it 

 forms the entire cUgeslive tract, but in most animals this is not suliicient 

 for the needs of digestion and the alimentary tract is increased by in\-ag- 

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



StomodEeum and Proctodffitmi. — Even in many cadenterates and 

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

 tract, forming the ectodermal fore-gut or sloinodicum (fig. 50). From the 

 higher worms onwards, it is accompanied by a second invai.dnation at the 

 hinder end, the ectodermal hind-gut, or proctodccnm (tig. 60) ; emljryolog- 

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

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

 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 

 proctod;eum and stomoda^um in making up the completed digestive tract, is very 

 different in the \arious 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 flie contrary, the ccloilermal portions arc 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 di\idcd into an anterior division, the hueeal 

 eavily, and a posterior one, the pliarynx. The narro^v tul)e leading from 

 this is the irsoplnii^iis (a) ; here and there it may vidcn, or bear a pouchlike 

 evagination, the erop or ■i]ii;liivies (/)), for the temporary reception of food. 

 From the oesophagus the food passes into a considerable enlargement, 

 the sloiiiiieh. Birds, like many other aninrals, htxxc a double stomach, 

 a thin-walled portion rich in glands, and a second part, the walls of which 



