THYONE 229 



2. The five radial canals (homologous with the radial 

 canals of the starfish and sea urchin) originate from the cir- 

 cular canal, pass forward and then backward, and end near 

 the cloaca. The radial canals take the general course of the 

 longitudinal muscle bands and lie between this muscle band 

 and the body wall. The radial canal may be seen if the 

 muscle band is carefully removed. 



3. Ten forwardly directed canals, the tentacular canals, 

 leave the radial canals near the circular canal and pass into 

 the tentacles, which may be homologized with tube feet. 



4. The stone canal and madreporite are much reduced in 

 holothurians. The madreporite, except in larvae and very 

 young specimens, is not found on the outer surface. The 

 stone canal leads obliquely backward from the circular canal, 

 toward the dorsal body wall, to join a small calcareous body, 

 the madreporite, which lies in the body cavity and is not 

 perforated. Does this give you a reason for the presence of 

 large Polian vesicles? The liquid in the water- vascular sys- 

 tem is not sea water. Notice its color. Are there cells in 

 it? Examine under microscope. 



Make a diagram of the water-vascular system. 



Muscular System. — Besides the special muscles radiating 

 from the cloaca which have been referred to in connection 

 with the respiratory system, and the muscles of the lantern, 

 there are five strong longitudinal bands, really pairs. In 

 which areas do they lie? What function do they perform? 

 Look for smaller circular bands. Are there many of them? 

 What is their function? Can you explain the varied worm- 

 like motions of the body by the action of these muscles? 



Nervous System. — This cannot be satisfactorily studied in 

 dissections. There are five radial nerves and a circular ring. 

 The nerves are embedded in the body wall and are hard to 

 find 



The classes of the Echinodermata show exceptionally well 

 how a general type of structure may be retained and still 



