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. Pedicellariz are absent. 
