HARD PARTS OF THE SEA-URCHIN'. 83 



other through a wide opening in the top of this partition 

 under each ambulacral plate. 



11. A lateral partition (e) under each ambulacral plate 

 carries a blood vessel out to the foot and ambulacral 

 vesicle. 



q. ^lake drawinsrs of the various sections, and trace out 

 the relation which the parts bear to each other in different 

 regions of the body. 



XII. THE EXA3IIXATI0X OF THE HARD PARTS 

 OF THE SEA-URCHIN. 



{Arbacia punctulata. ) 



SEA-URCHrvs may usually be obtained in abundance at 

 or a little below low-tide mark in sheltered places among 

 the fronds of algse. Either the purple urchin (Arbacia) 

 or the greenish brown one (Strongylocentrotus) may be 

 used for laboratory work. The former is more abundant 

 on the southern coast, while the latter is most easily pro- 

 cured on the Xew-England coast. 



The following description has been written from speci- 

 mens of Arbacia, but the differences between the two are 

 so sUght that either form may be used. Specimens should 

 be presened dry, and also in alcohol. 



I. In a specimen which has been dried without removing 

 the spines obsen'e the central spheroidal body, flattened 

 upon one side, and nearly covered by the long, pointed 

 spines. The flattened surface is the oral or actinal, and a 

 small area free from spines at the opposite part of the body 

 is the anal or ab-actinal area. On the oral surface, 

 notice : — 



a. The membraneous peristome, with small uTcgular 



