SEA CUCUMBER 157 
carefully their extent. It is by the periodic dilation and con- 
traction of the muscular walls of the rectum that water is alter- 
‘nately taken into the rectum and the respiratory trees and 
expelled from them, and respiration thus carried on. 
Exercise 2. Draw a diagrammatic view of the opened animal on 
a large scale, showing the organs observed. Carefully 
label all. 
Cut the esophagus just back of the calcareous ring and remove 
the digestive tract with the respiratory trees and the genital 
gland from the body. The five retractor muscles of the calca- 
reous ring may be followed to the body wall, where they will 
be seen to join five longitudinal muscles which extend along the 
inner surface of the body wall the length of the body. These 
muscles mark the five radial areas of the body. Note the circular 
muscles which also lie on the inner surface of the body wall. 
Study the ambulacral system. The ring canal, which is often 
difficult to see, surrounds the cesophagus at the hinder end of 
the calcareous ring. The Polian vesicles, two elongated sacs which 
have already been noted, extend from it into the body cavity. 
They secrete and store the lymphatic fluid which fills the ambu- 
lacral canals. The stone canal, which also extends from the ring 
canal, is a slender tube, and the madreporic plate with which it 
ends lies in the body cavity and not at the surface of the body. 
It is probably a rudimentary structure. The five radial canals 
extend from the ring canal along the radial areas of the body 
and beneath the five radial muscle bands to the hinder end of the 
body. The ambulacral feet extend from the radial canals through 
the body wall and are seen thickly studding the outer surface 
of the body. The numerous ampulle extend into the body cavity 
and will be seen on the inner surface of the body wall. The 
tentacles are also a part of the ambulacral system, the canals 
which supply them with the ambulacral fluid springing from 
the radial canals. 
