THYONE. 83 



ated. Does this give you a reason for the presence of large 

 Polian vesicles? The liquid in the water-vascular system is 

 not sea-water. Notice its color. 



Make a diagram of the water-vascular system. 



Muscular System. Beside 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 modi- 

 fied in certain regards for special habits. Compare, for instance, 

 the feeding habits of the starfish, sea-urchin, and sea-cucumber. 



