A SEA URCHIN 153 
The ambulacral system is similar to that of the starfish. A ring 
canal surrounds the cesophagus just inside the inner end of the 
dentary apparatus and is connected with the madreporic plate 
by means of the stone canal. This organ is a small tube which 
lies in contact with the cesophagus and also, in the neighbor- 
hood of the aboral body-wall, with the intestine. From the 
ring canal five radial canals pass along the median lines of the 
rays to their aboral ends, sending off branches to the ambulacral 
feet. The entire system of tubes, except the ambulacral feet, 
is within the body-cavity, instead of outside of it, as in the star- 
fish. Look on the inner surface of a ray for the radial canal. 
On each side of it observe the row of small vesicles, the ampulle, 
the reservoirs of the ambulacral feet. Determine the exact 
relation of the ampulle to the feet, and of both to the ambu- 
lacral pores in the shell. It will be seen that there are two 
rows of these latter on each side of the radial canal. Through 
one of these rows the branch canals pass from the radial canal 
to the ambulacral feet on the outside of the shell; through the 
other row projections of the feet pass back into the body-cavity, 
where they expand to form the ampulle. There is thus a single 
row of feet on each side of the radial canal in each ray. 
The ambulacral system will be seen to consist of a system of 
tubes extending throughout the body, and communicating with 
the sea water through the madreporic plate. It is filled with a 
fluid which, as in the starfish, is not pure sea water, but is 
rather a watery serum in which float amceboid cells. This fluid 
can be driven into the ambulacral feet, which acquire rigidity 
and are thereby extended. The animal moves by extending 
the feet, attaching the sucker discs at their ends to some sta- 
tionary object, and then drawing them in; it is able thus to 
pull itself slowly along. Some sea urchins with long spines 
also move on the tips of their oral spines as on stilts. 
Exercise 8. Draw adiagram representing the ambulacral system. 
