ECHINODERMATA. 39 



How -would you describe the exact location of the 

 madreporite ? 



!E. Drawings. 



1. A diagram of the aboral surface. (6 cm.) 



2. A diagram of the oral surface. (6 cm.) 



3. A drawing of a portion of the body wall to show the 



relation of the ossicles to the skin. 



4. A diagram to show the arrangement of the ambu- 



lacral feet. 



5. A careful drawing of the aboral surface. 



6. A similar drawing of the oral surface. 



2. THE SEA-URCHIN. 



A. Compare a sea-urchin and a starfish with regard to the 



following points, — 



(1) the shape ; 



(2) the symmetry ; 



(3) the surfaces ; 



(4) the spines. 



B. Exoskeleton. 



Notice that in the sea-urchin the exoskeleton is com- 

 posed of plates united in such a way as to form a 

 nearly spherical shell. (Dem.) 



1. Identify the areas of large perforated plates (aml)u- 



lacral plates). To what in the starfish do these 

 correspond with respect to position ? 



2. Identify the areas of smaller interambulacral plates 



between the ambulacral areas. 



3. How many sets of ambulacral and interambulacral 



areas are there ? 



