154 AN INTRODUCTION TO THE STUDY OF FOSSILS 



2. Give the habitat of Asterias. 



3 . How does it move ? 



4. What is its food ? Describe how it eats. 



5. How is the food digested ? assimilated? waste expelled ? 



6. What protection against enemies has Asterias ? 



7. Is the skeleton (test) external or internal? Explain. 



8. How does the animal respire ? 



9. Of what does the nervous system consist ? Its use ? 



10. Describe the sense organs. 



11. Give very briefly its development from the egg to the 

 adult. 



12. How long does it take to mature ? 



13. What makes the starfish such an enemy to oyster and 

 clam culture ? 



14. In what respects are the echinoderms more highly 

 evolved than the coelenterates ? 



CLASS A, CYSTOIDEA (CYSTOIDS) 



Calyx usually stemmed. Mouth nearly or quite central upon 

 the upper (ventral) surface. From the mouth radiate two to 

 five or more simple or branching ambulacra, along which passed 

 food particles to the mouth, probably driven by numerous cilia. 

 These food grooves may be upon the outer surface of the calyx 

 plates, between them, or upon their inner surface, and are rarely 

 extended into free branches, the arms. Anal opening excentric, 

 often closed by a valvular pyramid. 



Calyx plates usually irregularly arranged, varying in number 

 from thirteen to several hundred ; as a rule a few, or all of the 

 plates, are pierced by pores which are either irregular or arranged 

 in rhombic figures, with half of the rhomb on one plate and half 

 on an adjoining one ; the pores from opposite plates are united 

 by closed straight ducts which may have functioned in respira- 

 tion. 



Cystoids are the oldest and least specialized group of the 

 Pelmatozoa (which include cystoids, blastoids, and crinoids), 

 and probably represent most nearly the ancestral t3^pe of the 

 three classes. 



