56 ECHINODERMATA. 



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

 of the spines ? 



2. It consists of radiating arms 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 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 ex- 

 ternally only by the madreporic plate. Examine this plate with 

 a lens and determine its structure. 



4. On the oral surface, the mouth. Note its size and see if 

 it is provided with jaws of any kind. Would you expect jaws? 

 Why? 



5. Radiating from the mouth are the ambulacral grooves, 

 one on each arm. In these grooves are the 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 movable. 



6. 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 ambu- 

 lacral feet used ? How far can they be protruded ? 



2. Tear the starfish quickly from the bottom. Do any of 

 the feet remain behind ? Understand how they are attached. 



3. Place the starfish on its aboral surface and watch it turn 

 over. 



4. Find the thread-like dermal branchiae projecting through the 

 body integument. Are they withdrawn when touched? What 

 is their function? 



5. Stroke the starfish with a camePs-hair brush and notice 

 how the hairs are caught. Can you determine by what and 



