THE ORIGIN OF THE ANIMAL KINGDOM. 77 



was developed an animal with an outer and an inner 

 layer of cells, and a mouth opening, and that this 

 form has been the starting-point of most, if not all, 

 of the subsequent multicellular animals. We shall 

 call it the Gastraea, although it may not have been 

 exactly the same sort of animal as that to which 

 this name was originally applied. 



Divergence of Types. 



The next step in our history of animals was one of 

 great importance. The Gastraea became moulded into 

 types which foreshadowed the animal world of to- 

 day. Of the modifications of the Gastraea by which it 

 became changed into the various types of higher 

 animals, embryology and anatomy alone give us evi- 

 dence, and even here the evidence is clear only in a 

 few cases. There seems to be conclusive proof that 

 the Gastraea was the last stage which was shared in 

 common by the different groups of animals, and 

 possibly some groups branched off even earlier than 

 this typical Gastraea stage. From this point cer- 

 tainly a divergence took place which soon resulted 

 in the formation of a number of animal types which 

 we now recognize as the sub-kingdoms of animals. 

 The details of this divergence embryologists have 

 not yet fully mastered. The simplest case seemed 

 to be along the one line which gave rise to the cce- 

 lenterates. In this line of descent the original Gas- 

 traea attached itself by the end opposite the mouth, 

 and by then undergoing various small changes in 

 form, such as production of tentacles and elongation 

 of the body, produced the group of animals of which 



