INTERNAL STRUCTURE OF THE SEA-XJRCHIN. 91 



Xm. THE IXTERXAL STRUCTURE OF THE SEA- 

 URCHIN. 



{Arbacia Punctulata.) 



Carefully remove the spines from an alcoholic or a fresh 

 specimen ; place it, mouth uppermost, in a shallow pan 

 or a bowl, and pour on enough dilute acid to cover all but 

 the oral surface (two per cent, nitric acid will answer). 

 After it has remained in the acid lonor enouofh to remove 

 the calcareous matter from the integument, — from 

 twenty-four to forty-eight hours, — wash it thoroughly 

 with water. Place it in a shallow pan, with enough 

 water, or water and alcohol, to cover it, and cut, with a 

 pair of sharp-pointed scissors, through the integument 

 just outside of and entirely around the periproct, taking 

 care to cut no deeper than the integument. From the 

 edges of this circular incision make five radiating cuts 

 along the lines of the inter-ambulacral sutures down to 

 the oral surface. 



I. Turn back the five flaps, and notice the following 

 structures : — 



a. The five large sacculated reproductive glands (Fig. 

 39, a), reddish-brown in the female, and gi'ay in the male. 

 They occupy the upper ends of the inter-ambulacral areas, 

 and are attached to the integument in such a way that 

 each would ])e split into halves if .the five radiating in- 

 cisions were deep enough. 



b. On the middle line of the inner surface of each flap 

 notice the radial water tube (Fig. 40, i). 



c. On each side of this a single row of flattened, leaf- 

 like ampullae (Fig. 40, J). 



