40 STUDIES OF ANIMAL LIFE. 



3. THE SEA-CUCUMBEH. 



A. Compare with the starfish and sea-urcliin in regard 



to the following points, — 



(1) the shape ; 



(2) the symmetry ; 



(3) the surfaces ; 



(4) the exoskeleton. (Dem.J) 



B. Identify, — 



(1) the ambulacral feet ; 



(2) the tentacles around the mouth. 



C. Suggestive Questions. 



1. How do the habitats of the starfish, sea-urchin and 



sea-cucumber differ ? 



2. Why are there no fresh- water representatives of this 



group ? 



3. How did Louis Agassiz illustrate the way in which a 



starfish might theoretically have becoime changed 

 into a sea-urchin ? 



4. Why are starfish of interest to oyster fishermen ? 



5. How does the starfish eat ? 



6. What is the " water-vascular system " ? 



4. THE DEVELOPMENT OF AN ECHINODERM. 



A. Identify and draw the following stages in the devel- 

 opment of a young starfish or sea-urchin, — 



(1) the egg, a single cell ; 



(2) the two-celled stage ; 



(3) the four-celled stage ; 



(4) the eight-celled stage ; 



