CNIDARIA AS THE ONLY COELEXTERATA 



59 



transition from a life in benthos to a life in the pelagic zone, 

 i.e. to the zone of the free water (see Fig. 10). 



In this connection we are able to ascertain a very welcome 

 regularity; it enables us to find out the way along which the 

 change takes place. The rule is that this change occurs first 

 in the peripheral parts of the body of the animal, and only 

 later in its internal organs. The eight "antimeres" or ribs that 



Fig. 10. Scheme illustrating the transition of primarily bilateral 

 symmetry to other symmetries as consequence of a changed way 



of life. 



can be found in an otherwise swivel-round body of Cteno- 

 phora show a nearly perfect radially symmetrical distribution, 

 and yet at the same time some well-preserved traces of a for- 

 mer bilateral symmetry can be observed in the central parts of 

 their digestive organs which are frequently called the gastro- 

 vascular system (see Fig. 3.). Similar conditions exist in numerous 

 anthozoan polyps as was first noticed by palaeozoologists. 



In our study of Ctenophora, we meet with the problem how 

 Life in the free zone, i.e. in the pelagic zone, influenced the 



