^^^^ THE NEW EVOLUTION ©|^ 



is usually reinforced with numerous sharp spines, or 

 the tough and leathery skin filled with limy plates 

 or spicules, provides a very effective defense against 

 aggression. They are therefore relatively safe from 

 enemies. This enables them to live exposed upon the 

 bottom in those places where food is most abundant, 

 there to consume it at their leisure. Many of them 

 simply swallow mud which is rich in the dead remains 

 of plants or animals. 



Almost perfectly protected and living with their 

 food supply, or at least near it, rapid locomotion is 

 not an essential for the echinoderms. But they make 

 up for the absence of the capacity for rapid locomotion 

 in their ability to move equally well in any direction, 

 an ability not possessed by animal types with a defi- 

 nite head end at which the main sense organs are col- 

 lected as is the case with the active types in all the 

 phyla we have just considered. 



While one echinoderm, lacking all traces of a skele- 

 ton, lives freely suspended in the sea, a number bore 

 in mud more or less like worms, others have a more or 

 less definite head end in spite of their radial symmetry, 

 and still others, permanently or temporarily attached, 

 feed more or less after the fashion of the barnacles 

 and oysters, still the echinoderms as a group fill an 

 economic niche which is quite different from that 

 occupied by the vertebrates, the arthropods, the mol- 

 lusks or the annelids. 



There is no object in carrying this recitation fur- 

 ther. It is evident that a considerable part of the 

 economic range of each of the groups we have con- 



[12.2.] 



