A SEA URCHIN 119 



Class : Echinoidea 

 A SEA URCHIN 



Several species of sea urchins occur along the Atlantic coast, 

 the most famiUar 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 substances 

 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 Hfe 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 am- 

 bulacra! suckers which surround the mouth, and near them the 

 five groups of pincer-like pedicellariae. Observe the long, slender 

 stalks of these organs. Near the margin of the peristome are five 

 groups of ambulacral feet, which are in the ambulacral areas, and 

 also ten groups of short branched gills, which mark the inter- 

 ambulacral areas. 



