An Echinoid. 33 



AN ECHINOID 

 (Arbacia punctulata) . 



External Anatomy. Compare the oral and aboral 

 surfaces with the same of the starfish. Note that there are 

 five ambidacral areas, separated by five inter-ambulacral 

 areas. Both areas bear numerous regularly arranged spines. 

 Note the " ball and socket ' : joint that permits movement 

 of the spines. Are all the spines, of different parts of the 

 body, of the same general structure ? Where are they most 

 elongated ? 



The aboral region is centrally free from spines, and is 

 called the periproct, in contradistinction from the mem- 

 branous tract around the mouth, the peristome. Find ten 

 large ambulacral suckers on the peristome. Are pedicel- 

 larice to be found on the peristome ? On other portions of 

 the body ? How do they differ from the same organs of the 

 starfish ? The periproct will be found to be perforated at its 

 centre by the opening of the anus which is covered by 

 four triangular valves. 



Forming the periphery of the periproct, are five radiating 

 plates, the genital plates. Each genital plate will be found 

 to form the apex of the inter-ambulacral series, and to be 

 perforated by a small opening leading from the sexual glands. 

 One of the genital plates will be found to be larger than its 

 fellows and to appear more or less granular. It is the 



