200 



GENERAL ZOOLOGY 



of time taken to solve the problem, nor an increase in the 

 frequency of movements best fitted to solve it," hence no 

 learning. 



Another line of experimental work related to the regenera- 

 tion of lost parts. Ophiuroids readily break off pieces of 

 their arms (whence the name, brittle-star) and many 

 species commonly practise autotomy to escape from ene- 

 mies. If an arm is grasped, it is thrown off and left 

 to wriggle in the claws of the aggressor, while the ophiuroid 

 escapes. Missing parts are readily regenerated and this 



A B C D F 



FIG. 82. Echinodermata. A, crinoid; B, heart urchin; C, brittle star; 

 D, regular sea urchin; E, sea cucumber buried in mud; F, cake urchin, or "sand 

 dollar." 



was made the basis for experimental work. One investi- 

 gator maintained that the rate of regeneration was propor- 

 tional to the degree of injury (i.e., the more cut off, the faster 

 the growth to replace the lost part), but further experiments 

 showed that such a ratio would not hold absolutely. 



CLASS 3. ECHINOIDEA 



The sea-urchins differ from all other echinoderms in 

 possessing (1) an immovable calcareous test, which is 



