126 PHYLUM ECHINODERMATA 



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 madreporite 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 ampullae 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. 



Exercise 3, Draw a diagram of the ambulacral system. 



Exercise 4. Draw a diagram representing an ideal cross section 

 through the middle of the body. 



The calcareous spicules and plates which form a part of the 

 body wall and are so characteristic of the echinoderms as a group 

 are less conspicuous in holothurians than in any of the other 

 classes of the phylum, and will not be studied in this dissection. 

 They are of minute size, although definite in shape in the various 

 species, and do not form a connected skeleton as in the starfish 

 and the sea urchin. Pedicellariae are absent. 



