158 INVERTEBRATE ZOOLOGY 



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. Pedicellariee are absent. 



