CHAPTER LI. 



THE DIGESTIVE SYSTEM 



It will be remembered that all multicellular animals pass through a 

 blastula stage consisting of a hollow sphere composed of a single layer 

 of cells, which then indents to form a gastrula. 



This means that there are now two layers of cells where there was 

 only one before. The outer layer is called the ectoderm and the inner 

 the endoderm. The indented end closes up, leaving a hollow tube com- 

 posed entirely of endoderm in the center, which, due to its being used 

 for other purposes than the ectoderm, and lying within the body, under- 

 goes totally different experiences than does the outer part of the body, 

 and these different experiences modify its structure. This hollow tube 

 is the primitive digestive tract. It will thus be seen that the digestive 

 apparatus is the very first one of the various systems of an organism 

 to differentiate. 



This distinctive cavity is called the gastrocoele. In the lower in- 

 vertebrates this gastrocoele remains as a blind cavity with but a single 

 opening. It is among the worms that it first becomes converted into a 

 complete canal by the formation of an anal opening. In animals up to 

 this stage the same opening serves both for ingestion and egestion. 



What is considered a distinct advance in the development of multi- 

 cellular animals is the development of a coelom, or body cavity, lying 

 between the digestive tract just mentioned, and the body wall. Up to 

 the time this coelom has developed, the body of the animal consists of a 

 single tube and its wall. But after the coelom has developed there is 

 established a secondary open space between the hollow digestive tube 

 and the body wall. 



The coelom is developed by protrusions or diverticula pushing off 

 from the original digestive tube (Fig. 426). This means that the diges- 

 tive canal of the higher animals only represents a portion of the digestive 

 system of lower animals. 



Another departure from the lower organisms consists in the fact that 

 the mouth and anal opening are not developed in the same way in the 

 vertebrates as they are in the lower forms of animals. 



In the lower forms, after gastrulation, the indented end remains 

 open, thus serving as both mouth and anal opening at the same time. 

 In the higher forms, however, this indented end closes so that there is 

 a completely closed hollow tube composed of endoderm on the inside of 

 the body. To form the mouth and anal opening a new indentation at 

 both the cephalic and caudal ends takes place. 



This indentation coming from the outer layer of the body means 



