ANIMALS WITH SPINY SKINS— THE ECHINODERMS 299 



Class B. Ophiuroidea (Gr. opliis, snake; oara, tail; eidos, like; their 

 arms resemble snake tails). The brittle stars. 



Class C. Echinoidea (Gr. echinos, spiny; eidos, like). The sea ur- 

 chins and sand dollars. 



Class D. Holothurioidea (Gr. holothnrion, sea cucumber ; eidos, like). 

 The sea cucumbers. 



Class E. Crinoidea (Gr. crinon, lily; eidos, like). The sea lilies. 



REVIEW QUESTIONS 



1. Why are the echinoderms considered more closely related to the chordates 

 than to any invertebrate phyla ? 



2. Why is the larva of the starfish of particular significance? 



3. What are pedicellariae and what is their function ? 



4. Describe respiration, circulation, and excretion in the starfish. 



5. Explain how tube feet are extended and retracted. 



6. Describe the feeding habits of the starfish. 



7. What is meant by autotomy? 



8. Why do oyster fishermen no longer chop up starfish and throw them back 

 in the water? 



9. Why has the retention of a bilateral, free-swimming larva been of survival 

 value to the starfish? 



10. Has the development of radial symmetry in the adult been of any value to 

 the starfish? If so what? 



11. What unusual method does the sea cucumber have of defending itself? 



12. Why are sea lilies more commonly seen as fossils than as living animals? 



13. In general, organs which are not used tend to deteriorate in the species to 

 the point where they become nonfunctional. In what way is this principle illustrated 

 in the starfish? 



