CHAPTER XIV. 



PHYLUM IV.-ECHINODERMATA (STAR-FISH, SEA-URCHINS, SAND- 

 DOLLARS, SEA-LILIES). 



LABORATORY EXERCISES. 



233. Asterias (Star-fish). Both dry and alcoholic, or 

 otherwise preserved, materials should be at hand. 



1. General form. 



Central disc. 



Rays; number, form, size, etc. Compare several in- 

 dividuals. 



Oral surface (contains mouth) ; aboral surface. Note 

 all the differences between these surfaces, both 

 in the arms and the disc. 



The axis of an arm is known as a radius; the space 

 between is interradial. 



Is the body bilaterally symmetrical or radially sym- 

 metrical? Give the reasons for your conclusion. 



2. External anatomy. 

 Oral surface. 



Mouth : position and surroundings. 



Ambulacral groove: position, relation to the mouth, 



extent. 

 Ambulacral feet: how arranged? Is the foot hollow 



or solid? Pull off one, and examine with lens or 



low power of the microscope. 

 Aboral surface. 



Madreporic body: position (radial or interradial?), 



shape, size, structure. 

 Bivium; trivium (see text, 237). 

 Examine the spines on both surfaces and determine 



the arrangement and shape in different regions. 



How are they fixed to the body? 



200 



