80 LABORATORY DIRECTIONS 



I. EXTERNAL CHARACTERS 



1. Observe the oral surface (the surface with the mouth). 



2. The aboral surface. How do the two surfaces differ? 



3. The central disc, from which the arms or rays radiate. 



4. Find the sieve plate or madreporic plate, a small round 

 area without spines, on the disc between the two rays which 

 form the bivium. Examine the madreporic plate with a hand 

 lens. Draw X5. 



5. Opposite the madreporic plate is the anterior ray, which 

 with the one on each side of it forms the trivium. 



6. Note the spines scattered over the surface of the animal. 

 Are they uniform in distribution? 



7. The anus, or external opening of the digestive system, 

 is located upon the disc. It is a vestigial organ and can not or- 

 dinarily be found. 



8. The fleshy protuberances between the spines, the gills, 

 are hollow finger-like processes which connect with the body 

 cavity and in which the body fluid circulates. Sea water bathing 

 the outer surface of the gills furnishes the necessary oxygen to 

 aerate the fluid within. 



9. The pedicellariae, small pincer-like organs, protect the 

 animal, and especially the gills, from injury and remove debris. 

 They are especially abundant about the bases of the spines. Find 

 them. Examine a prepared slide of pedicellariae under the mi- 

 croscope. Find and draw two distinct kinds. In the larger type 

 the basal portion is a distinct piece upon which the two jaws are 

 hinged while in the smaller type the basal parts of the two jaws 

 cross over each other like the handles of a pair of shears or 

 tongs. 



10. A line drawn from the center of the disc along the 

 middle of an arm is a radius. One drawn from the same point 

 exactly between two arms is an inter-radius. Do all radii inter- 



