Echinodermata. 



333 



will be seen that these tube feet are retracted, extended, 

 and variously moved about. For protection the feet can 

 be withdrawn into the groove. But commonly the feet 

 are applied to the surface on which the starfish is creep- 

 ing. Each tube foot is a hollow cylinder, containing 

 water, and is connected by a slender tube, passing be- 

 tween the ossicles, with a water bulb in the cavity of the 

 ray. Both the foot and the bulb are muscular, so when 

 the bulb contracts it forces more water into the foot and 



.Madreporic 

 plate 



Stone canal 



Radial canal 



FIG. 189. WATER TUBE SYSTEM OF STARFISH. 



extends it ; and when the foot shortens it can send the 

 water back into the bulb. Around the mouth is a circular 

 water tube, which sends a tube along each ray ; and side 

 branches from these radiating tubes supply the tube feet. 

 On the outside of the aboral surface, between the bases of 

 two of the rays, there is a wartlike body. This is the 

 madreporic plate. It is perforated. Water enters it and 



