PHYLUM ECHINODERMATA 



193 



nected with a little reservoir, or ampulla, within the body 

 wall. Muscles in the wall of the tube-foot make it possible 

 to move the organ about, like an elephant's trunk, and the 

 adhesive disc may thus be put down in a particular spot. 

 When the tube-foot is extended the reservoir connected 

 with its base is small, but if it contracts, the reservoir is 

 distended by the water which is forced into the body. 

 Water first enters the starfishes' water-vascular system 

 through the madreporic plate, or " sieve plate," passes 

 through a straight stone canal to a ring canal around the 

 mouth, and from this a radial vessel which supplies the 



B c 



FIG. 80. Starfishes. A, eating an oyster; B, an individual which has lost 

 four rays and is growing them anew; C, crawling by using the tube-feet. 



tube-feet runs down the under side of each arm. The 

 broad spread of the body and the great number of tube- 

 feet enable starfishes to walk over soft mud, and the ad- 

 hesive discs permit them to climb up vertical surfaces with 

 ease. Though able to move but slowly, starfishes are well 

 equipped to seek for food. 



A starfish can tell when substances are present in the 

 water which indicate that food is near. If an individual is 



