ASSIMILATION 



47 



a clear spot that alternately appears as it becomes filled 

 with fluid and disappears as the fluid is emptied into the 

 water outside. 



In Many-Celled Animals. — The simple process in 

 one-celled animals has all the essential features that char- 

 acterize higher forms. In many-celled animals an organ 

 is set apart for the reception of food. As the cells in 

 the young animal begin to develop they become arranged 

 in the form of a hollow sphere. At one point on the sur- 

 face of the sphere the cells push in until they reach the 

 opposite surface. This structure, called the gastrula, 

 is bag-like. The wall is made of two layers of cells and 

 the cavity opens to the outside by a single opening. The 

 cells on the inside soon grow different from those on the 



Fig. 29.— Formation of gastrula. (From Hertwig, after Hatschek.) 



outside and become adapted to the assimilation of food. 

 This is the primitive digestive tract (Fig. 29). 



A great number of animals are still in the gastrula 

 stage. Though they differ widely from each other, they 

 are classed together, for they have one characteristic in 

 common. The single cavity in the body is adapted to 

 the assimilation of food and it has but one opening to 

 the outside. Through this opening food enters and unfit 



