90 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



tube, the latter being inside the shell of the sea-urchin. 

 The other pore, the one nearest the inter-ami mlacral 

 suture, is that through which each foot is connected with 

 its ampulla. The growth of the ambulacral plates shows 

 that each one of them is compound and really consists of 

 as many separate plates as there are pairs of ambulacra! 

 pores. With care the lines between these smaller plates 

 may be made out, as shown in Fig. 38, and they remain 

 distinct through life around the mouth-penta- 

 gon, as shown in Fig. 37. 



FIG. 38. Five ambulacral plates from the point 

 marked "A," in Fig. 36, more enlarged, to show the 

 SM ondary plates of which they are made up. (Drawn 

 FIG. 38. from nature by Mr. H. J. Rice.) 



7i. Make a sketch of a few ambulacral plates, showing 

 as many of these points as possible. 



o. Notice that each occular plate is at the end of a series 

 of ambulacral plates, as in the starfish, while the ovarian 

 plates, like the reproductive orifices of the starfish are oppo- 

 site the inter-ambulacral sutures. 



p. Make a sketch of the alvoral surface of the corona, 

 showing all the plates and sutures : and indicate in the 

 sketch the limits of the five rays. 



q. Make a similar sketch of the oral surface. 



r. Notice the five pairs of auriculae, or perpendicular 

 plates, which project upwards from the inner surfaces of 

 the ambulacral plates which form the sides of the mouth- 

 pentagon. 



