34 



EVOLUTION AND GENETICS 



the Protozoa most are independent single cells, but some are asso- 

 ciated together in groups (Fig. 15), and most of them remain un- 

 differentiated. The next three phyla are 

 fundamentally simple two-layered sacs 

 although they are modified in various 

 ways, and are called diploblastic. One 

 little fresh-water animal. Hydra, shows 

 this arrangement very simply (Fig. 16). 

 All other phyla have three fundamental 

 layers, and are called triploblastic. Their 

 relationships are indicated by the de- 

 velopment of other structures. 



Triploblasts. Symmetry. Complexity 



Fig. 15.— a simple colony ^^ ^^^^ environments to which these ani- 



of unicellular organisms mals are adapted is here reflected in a 



(^poudi^lnmornm). (From variety of different tendencies in their 



Hegner, atter Oltmanns.) . , t.t . i, ,. , , . ,, 



structure. Not all of them have followed 



the same course of development beyond this acquisition of three 

 fundamental germ layers. Thus, the flat worms have merely 

 added to the complexity 

 of the original sac-like 

 digestive cavity, while 

 the rest have developed 

 a tubular structure, open- 

 ing cephalad at the 

 mouth and caudad at the 

 anus. Yet the flat worms 

 share with the remaining 

 phyla the development 

 of a head in which nerv- 

 ous control is concen- 

 trated, and accompany- 

 ing this, their bodies are 



formed of two similar 



halves, flanking the me- Fig. 1G. — Hydra. Transverse section, highly 

 dian axis. Below this magnified. (From Woodruff, after Shipley 



.'. . , and McBride.) 



group, animals are either 



asymmetrical or made up of parts radiating from a common 

 center, as in Hydra. Such a form is known as radially symmet- 

 rical, while the bilateral arrangement is called bilateral symmetry. 



