THE ORIGIN- OF THE ANIMAL KINGDOM. 7 1 



in the history of animals, and from this one all other 

 types of animals have developed. 



The early type of animal thus referred to as of so 

 much importance in the history of living things is 

 one of which we have abundant evidence in embry- 

 ology, though perhaps the type does not exist to-day 

 as a distinct animal. Since embryology is the only 



FIG. 6. A Typical Gastrula. The shaded cells, en, are those connected with 

 the digestion of food, en Endoderm. ec Ectoderm. 



evidence of this stage in the history of life, our 

 knowledge of this early form is, for reasons shown, 

 confined to its fundamental structure with practically 

 no sure knowledge of its details. The type in ques- 

 tion has been named the Gastraea by Haeckel, who 

 first clearly saw that embryology taught that such a 

 common ancestor formerly existed. The evidence for 

 this conclusion is as follows : It has been found by a 



