206 



MARINE INVERTEBRATES 



bases of the spines, and form groups over the dorsal surface. 

 Examining the backs of different starfishes with a strong magni- 

 fying-glass, one will be surprised to see the varied grouping of 

 the spines and pedicellariae. At the extremity of each arm is a 

 red eye-spot, which is sensitive to light, and a tube-foot, which has 



no sucker and which is believed to 

 act both as an olfactory and as a 

 feeling organ. The madreporic plate 

 lies at the angle 

 of two of the 

 arms or rays. 

 Water filters through the 

 madreporic plate and 

 passes by the stone-canal 

 to the ring-canal around 

 the mouth, and thence to 

 the radial tubes, one of 

 which extends through 

 each arm and is connected 

 by a branch with each 



Diagram of water-vascular system of a starfish : a, , ^ , , 



madreporite ; b, stone-canal ; c, ring-canal ; d, radial canals ; LUDe-IOOt. -A.U tne DaSC Ol 

 e, ampulla,;/, ambulacra. each tube . foot there ig a 



globular reservoir, or ampulla. A valve connects the tube-foot and 

 ampulla. When the ampulla is contracted, water is forced into and 

 distends the tube-foot ; when the ampulla is distended, it withdraws 

 the water from the tube-foot, thus acting like a suction-bulb. When 

 water is sent into the tube-feet they are greatly extended and are then 

 placed against an object. The water being then withdrawn through 

 the ampullae, a vacuum is formed, and the suckers are tightly at- 

 tached. By the alternating movements of the ambulacra the 

 animal is dragged slowly along and moves with a gliding motion. 

 It does not deviate from a straight path, going over elevations 

 or through depressions without turning aside ; its body con- 

 forms to the irregularities of the road and never bridges over 

 spaces. The stomach extends a little way into the arms, and a 

 short intestine leads to the excretory opening in the center of the 

 back. Starfishes are carnivorous and voracious, and are partic- 



