70 ECHINODERMATA. 



Examine a specimen and notice: 



1. That the surface by which the animal chngs, the oral 

 surface, is different from the other, ahoral surface, and that 

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

 of the spines? 



2. It consists of radiating artns and a central disk. 



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

 two arms, a small, frequently conspicuously colored, circular body, 

 the 7nadreporic plate. The two arms adjacent to this plate are 

 sometimes referred to as the hivium, and the remaining three as 

 the trivium. The radial symmetry of the animal is disturbed ex- 

 ternally 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 the ambulacral or tube feet. Do they have a definite arrange- 

 ment? Along the sides of the grooves are slender spines that 

 differ from the general body-spines in being movable. 



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 ex- 

 posed 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 co-ordinated movement? 



2. Place the starfish on its aboral surface and analyze the 

 method of righting. 



3. Tear the starfish quickly from the substratum upon which 

 it is crawling. Are any of the feet torn from the animal? (See 

 Paine for a study of the adhesive power of the tube-feet.) 



4. Find the thread-like dermal branchiae projecting through 



Paine, V. L.: Adhesion of the Tube-feet in Starfish. J. E. Z., vol. 46, 

 No. 2, 1926. 



