A SEA URCHIN 149 
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 
