76 ECHINODERMATA. 



body-cavity. It is composed of a multitude of filaments, which 

 join to make a brush. This brush projects backward into the 

 body-cavity. The duct of the organ lies along the dorsal mid- 

 line, between the right and left dorsal muscle bands, and leads 

 to the opening upon the small papilla near the mouth that has 

 already been noticed. 



Water-vascular System. 1. The circular canal can be found 

 in favorable specimens, surrounding the deeper portions of the 

 esophagus. It gives rise to one or two Polian vesicles, which 

 are very large and hang down into the body-cavity. 



2. The five radial canals (homologous with the radial canals 

 of the starfish and sea-urchin) originate from the water-ring, 

 pass forward and then backward, and end near the cloaca. The 

 course of each radial canal is easily followed by means of the 

 numerous small, elongated ampullae which supply the tube feet. 



3. Ten forwardly directed canals, the tentacular canals, 

 leave the radial canals near the water-ring 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 water-ring, 

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

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

 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? 



