ASTERIAS 213 



mens can be found with pieces of grass and weeds covering 

 them. Try picking these pieces off to see if they adhere. 



5. Do the animals have other means of protection? 



Examine a specimen and notice that: 



1. The surface by which the animal clings, the oral sur- 

 face, is different from the other, aboral surface, and both 

 surfaces are covered with short spines. What is the use of the 

 spines? 



2. The animal consists of a central disk and radiating 



arms. 



3. On the aboral surface of the disk, near the junction of 

 the two arms, is a small, frequently conspicuously colored, 

 circular body, the madreporic plate. The two arms adjacent 

 to this plate are sometimes referred to as the bivium, and 

 the remaining three as the trivium. The radial symmetry of 

 the animal is disturbed externally only by the madreporic 

 plate. Examine this plate with a lens and determine its 

 structure. 



4. Radiating from the mouth situated on the oral surface 

 are the ambulacral grooves, one on each arm. In these 

 grooves are ambulacral or tube feet. Do they have a definite 

 arrangement? Along the sides of the grooves are slender 

 spines that differ from the general body spines in being mov- 

 able. 



5. Scrape the tube feet from a portion of an ambulacral 

 groove of a dried specimen and notice the pores through which 

 the feet are attached to organs inside the arm. Notice also 

 the exposed ambulacral plates and determine their relation 

 to the pores. 



Draw figures of the aboral and oral surfaces of a starfish, 

 and a diagram to show the relation of the ambulacral plates 

 and pores. 



Place a living starfish in a dish of sea water. 



1. Study its method of locomotion. How are the tube 

 feet used? Does each foot act independently, or is there any 

 evidence of coordinated movement? 



