BRANCH ECHINODERMATA : STARFISHES, ETC. 115 



tain small pigment-cells and communicate with the 

 nervous system. 



From a general inspection of the sea-urchin's shell the 

 Echinoderm characteristics, namely, radial symmetry and 

 the presence of the water-vascular system, are readily 

 seen. While at first glance there is apparent little 

 similarity between the starfish and sea- urchin, neverthe- 

 less careful examination shows that the two animals are 



FIG. 2i. "Test" of Sea-urchin, Strongylocentrotus franciscanus, with 

 spines removed. (From specimen.) 



alike in their fundamental structure. Both are radially 

 symmetrical. The position of the anal opening makes 

 both starfish and sea-urchin slightly asymmetrical. In 

 both the madreporite and anus are on the aboral side, 

 while the mouth is centrally located on the oral side! 

 In the starfish we noted five ambulacral areas, one on the 

 under side of each arm ; similarly we find five in the sea- 

 urchin. In both cases also we find the ocular spots at 

 the tips of the ambulacral areas. The genital apertures 

 are situated interradially in the starfish. In the sea- 

 urchin they are similarly placed. The dissimilarity 

 between the two forms is largely due to the very much 

 developed outer spines and the dorso-ventral thickening 

 of the disk in the sea-urchin. The starfish is carnivorous, 

 while the sea-urchin lives on vegetable matter consisting 



