THE STARFISH AND SOME ALLIES 247 



the resemblance striking. In Pentacrinus blakei the number 

 of arms varies ; it may be as great as twenty. The sea-lily 

 is different from the starfish and the basket-fish, in having a 

 long stalk which grows from the center of the aboral surface 

 and sends root-like branches in among the rocks at the bottom. 

 The mouth and the arms lie on the oral surface, which is 

 uppermost. A water-vascular system is present, but it is of 

 no service to the animal in locomotion. 



Movement in Pentacrinus is limited to the arms. A related 

 genus, Comat'ulus, found off the North Atlantic coast, is fixed 

 by a stalk when young, and free in its adult stage, moving 

 about then like a basket-fish. Food is brought to the mouth 

 of Pentacrinus by the wave-like action of cilia on the inner 

 edge of all the arms. 



THE SEA-URCHIN 



Practically any tide-pool along the North Atlantic coast that 

 affords specimens of starfish will harbor several sea-urchins. 

 The geographical distribution of the species Strongylocentro'- 

 tus drobachien'sis (Fig. 124) is very wide. It is found on the 

 coast of Great Britain and Norway, along the North Atlan- 

 tic coast, and also on the North Pacific coast, in all these 

 regions from tide-water down to several hundred fathoms. 



Deprived of its spines a sea-urchin suggests, as Professor 

 W. F. Ganong aptly says, " an old-fashioned door-knob." It is 

 flattened on the oral surface and curved above into a rounded 

 dome. From the center of the dome one may trace downward 

 twenty radiating rows of calcareous plates, fitting against one 

 another closely. Five pairs of these plates radiating 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 



