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 pedicellaria. 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 
dot 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 
e by a branch with each 
saadripecite, 6, Hone canel;e ragennaly4,retuleanaia, tube-foot, Ab the base of 
ha ati daca each tube-foot there is a 
globular reservoir, or ampulla. A valve connects the tube-foot and 
ampulla. Whenthe 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 The water being then withdrawn through 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 ampulle, 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- 
WN BY rg): 
