THYONfi. 69 



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. Ten forwardly directed canals leave the water-ring and 

 pass into the tentacles, which may be homologized with tube feet. 



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

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

 and pass backward, ending near the cloaca. The course of each 

 radial canal is easily followed by means of the numerous small, 

 elongated ampulla which supply the 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 body- 

 wall, to join a small calcareous body, the madreporite, which 

 lies in the body-cavity. Does this give you a reason for the pres- 

 ence 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. 



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. 



