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 oesophagus 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 oesophagus 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 ampulla? 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. 



