HARD PARTS OF THE SEA-URCHIN. 



87 



e. Examine the oral surface, and notice : 

 1. The large mouth-pentagon which forms nearly the 

 whole oral surface of the corona, and is normally closed 

 by the peristome. The aperture is pretty definitely pen- 

 tagonal ; the angles are concavely rounded, and there is a 

 deep notch in the middle of each side. 



FIG. 37. 



Fir,. 37. The oral surface of the corona of Arbacia, a little enlarged. 

 (Drawn from nature by Mr. H. J. Rice, under the author's direction.) 

 The letters and figures of reference are the same as in Fig. 36. 



2. The five inter-radial sutures (Figs. 36 and 37, d} : zig- 

 zag lines which run around the corona from the tip of each 



