THE STARFISH AND SOME ALLIES 



233 



Practically any tide pool along the north Atlantic coast 

 that affords specimens of starfishes will harbor several 

 sea urchins. The geographical distribution of the species 

 Strongylocentro'tus drobachien' sis (Fig. 126) is very wide. 

 It is found on the coast of Great Britain and Norway, along 

 the north Atlantic coast, and also on the north Pacific coast, 

 — in all these regions 

 from tidewater down 

 to several hundred 

 fathoms. 



If deprived of its 

 spines a sea urchin 

 suggests, as Professor 

 W. F. Ganong has 

 aptly said, "an old- 

 fashioned door-knob," 

 for it is flattened on 

 its oral surface and 

 curved above into a 

 rounded dome. From 

 the center of the dome 

 one may trace down- 

 ward twenty radiating rows of calcareous plates, fitting 

 against one another closely. Five pairs of these plates radi- 

 ating at equal angles have many fine holes for the tube feet ; 

 five other pairs of plates lie in the spaces between the regions 

 of tube feet. All over the aboral surface and over most of 

 the oral surface, the plates bear short, rounded knobs on 

 which the spines fit and form a ball-and-socket joint. On 

 each tube foot area at the point nearest the center of the 

 aboral surface lies an eye; and between every two eyes 

 there is an opening for the eggs or spermatozoa to emerge. 

 A sieve plate lies in one of the spaces between two eyes. 

 The mouth has five sharp teeth which meet together in a 



Fig. 126. Photograph of a sea urchin. 

 (Slightly reduced) 



