PHYLUM ECHINODERMATA 199 



gression. The embryological evidence (Fig. 81) indicates, 

 according to the Law of Biogenesis, that starfishes were 

 once free swimming bilaterally symmetrical animals (Fig. 



81, H), with all the possibilities that go with that type 

 of bodily arrangement, and that they gave up certain 

 possibilities to become radially symmetrical and unprogress- 

 ive. It is believed that this change came about because 

 echinoderms at one time in their racial history took up 

 a sessile mode of life and grew attached to the ocean bottom. 

 It is desirable for an attached animal to be able to receive 

 stimuli. and capture food from as many directions as possible, 

 hence the assumption of radial symmetry. But echino- 

 derms, as a race, made a serious mistake when in the 

 course of evolution they came to an attached mode of 

 existence, for no sessile animal can progress very far. 

 There are now some points of actual degeneracy. For ex- 

 ample, the anus of the larva is functional (and was probably 

 so in the ancestral types) , but in many adult echinoderms is 

 not used or is absent. In modern times there have been 

 evolutionary attempts to correct early racial errors: 

 most echinoderms, like the starfish, have taken up a free- 

 living existence; some sea-urchins and sea-cucumbers have 

 even become bilaterally symmetrical as adults (Fig. 



82, B, F). 



CLASS 2. OPHIUROIDEA 



The brittle-stars, or snake-stars (Fig. 82, C) are very 

 active, being able to bend the solid arms readily and thus 

 swim about. They feed mostly on minute organisms 

 which they scrape from marine plants with their feet. 

 No anus is present. 



On account of their activity, ophiuroids have been much 

 used for experimental work. Attempts to teach them 

 simple lessons have met with little success. In one series 

 of experiments a rubber tube was slipped over one of an 

 ophiuroid's arms, and behavior in attempting to remove it 

 observed. There was " neither a decrease in the amount 



