A SEA URCHIN 139 



ECHINOIDEA 



A SEA URCHIN 



Several species of sea urchins occur along the Atlantic coast, 

 the most familiar being Arbacia, the dark-colored urchin, and 

 Strongylocentrotus, the green urchin, the former having a more 

 southerly distribution than the latter. The animals live on the 

 sea bottom or on rocks, usually in companies, and move slowly 

 about from place to place, using not only the ambulacral feet, 

 but often the spines as well, as organs of locomotion. They 

 feed partly upon small animals and partly upon organic sub- 

 stances present in the sand and mud, which they pass through 

 the intestine. 



Two specimens will be needed for this dissection, a dried one 

 for the study of the hard parts and a fresh or preserved one for 

 the study of the internal organs. 



Observe the radiate spheroidal body entirely covered with 

 movable spines. Look among the spines and find the ambulacral 

 feet. These can be extended in life beyond the spines and are 

 employed by the animal as organs of locomotion. Note the five 

 ambulacral areas (those containing the feet), and between them 

 the five interambulacral areas. The flattened surface is the under 

 or oral surface, on which the animal moves ; the rounded surface 

 is the aboral. It will be seen that the aboral side of the sea 

 urchin bears ambulacral feet, whereas in the starfish the oral 

 side alone bears them. 



In the center of the oral surface, observe the mouth and the 

 five calcareous teeth which project from it. Surrounding the 

 mouth is a membrane which fills the space between the edges 

 of the shell and is called the peristome. Notice the ten short 



