214 ELEMENTARY ZOOLOGY. 



Not attached. 



Oral-aboral axis short and usually vertical. 

 Body with arms extending from a central disc. 



Arms with ambulacra! groove Class Asteroidea. 



Examples: Starfish; Asterias, Solaster. 



Arms without an ambulacral groove . . .Class Ophiuroidea. 

 Examples: Brittle stars; Amphiura, Astropkyton. 

 Spherical or flattened body with no free arms. Class Echinoidea. 

 Examples: Sea-urchins and sand- dollars; Echinus, Strongilo- 

 centrotus, Clypeaster. 

 Oral-aboral axis elongated, usually horizontal. 



Class Holothuroidea. 

 Examples: Sea-cucumbers; Holothuria, Cucumaria, Pentacta. 



234. Topics for Themes in Zoology and English. i. 



Report on the habits, appearance, and abundance of 

 the crinoids in geological times. 



2. Define autotomy. What is known of its occurrence 

 in this group? Of what possible advantage would this 

 power, coupled with power of regeneration, be to a species? 



3 . Study the echinoderms as a group that has degener- 

 ated from its original condition. What are its present 

 adaptations to a sluggish life? 



4. From a study of figures in the texts and of specimens 

 in museums, find a tendency on the part of some sea- 

 urchins to become bilaterally symmetrical in the adult. 



5. Write an account of the protective structures on 

 the surface of echinoderms, and of their probable func- 

 tions, including spines and pedicellariae. 



6. Compare representatives of each of the five classes 

 in the following respects: the nature of the mouth and 

 the parts of the digestive tract ; the powers of locomotion ; 

 respiration, and the metamorphosis. 



7. The nervous system and the organs of special sense 

 in echinoderms. 



