REVIEW OF CLAM, SNAIL, ETC. 153 



water-pipe system, by means of tube feet. The whole 

 water-pipe system is filled with water which enters 

 through the madreporic body, passes through the 

 stone canal, through the circum-oral water ring, 

 through the radial water tube into the ampullae. 

 Here the contraction of little muscles forces the water 

 into the tube feet, extending them to their full length. 

 When the tip of one of the tube feet is pressed against a 

 flat surface, the lifting of the central portion of the 

 disk at the tip creates a vacuum which fastens that 

 foot to the surface. As the tube foot then shortens, 

 the ray to which it belongs is drawn closer to the 

 point of attachment. By the action of many tube 

 feet, the starfish progresses, although it moves exceed- 

 ingly slow. 



The starfish has the power to reproduce lost parts 

 in a very marked degree. If all of the rays except 

 one be pulled off, the starfish will still live, and it 

 will soon reproduce the missing rays. 



There are no specialized breathing organs. The 

 water-pipe system performs the function of res- 

 piration. 



The pedicellariae are small, pincer-like organs sur- 

 rounding some of the spines on the aboral surface. 

 Their function is not fully known. 



Kevlew of Clam, Snail, Eartliworm, and Starfish. 



The Clam. 



1. What are the valves? 



2. How distinguish the right and the left valve? 



3. What is the umbo? 



4. What is the hinge ligament? 



5. What is the mantle? 



6. What are the palpi? 



7. How many gills has the clam? Explain their 



structure. 



