4 HlCKSON, Animal Symmetry. 



anemone and the solitary coral. A solitary coral such as 

 Caryophyllia is permanently fixed to a solid object at the 

 bottom of the sea and obtains its food by seizing with its 

 tentacles the prey that floats or swims within their reach. 

 We may suppose that in shallow sea water with its con- 

 stantly shifting currents food may arrive within reach from 

 any point of the compass and the radial arrangement of 

 the tentacles is the most efficient arrangement for catching 

 the food at the earliest possible moment. 



In Hydra and in the Sea-anemone there is some 

 power of movement from place to place although the 

 feeding habit is essentially that of sedentary animals. 

 Hydra is perfectly radially symmetrical, with a cylindrical 

 body, a round mouth and a ring of tentacles, but in the 

 sea-anemone there is a subordinate bilateral symmetry 

 shown in the slit-shaped mouth and certain details of the 

 structure of the mesenteries. It would be an interesting 

 study to inquire into the origin of this subordinate 

 bilateral symmetry in the sea anemones and other zoan- 

 tharia, but, beyond expressing the opinion that it 

 originated at a time when the power of movement of 

 ancestral zoantharia from place to place was greater than 

 it is now, the matter will not be further discussed in this 

 address. 



The radially symmetrical form of animal shape, then, 

 is the form that is best adapted for an animal that is 

 fixed to the bottom in still water, or in water with variable 

 currents from all directions. But it is also the form best 

 adapted for an animal that floats or drifts about at or 

 below the surface of the water, and we find numerous 

 examples of perfectly radially symmetrical forms of 

 Heliozoa, Radiolaria, and Foraminifera among the Pro- 

 tozoa as well as of Medusae among the Coelenterata and 

 in the group of the Ctenophora. 



