ECHINODERMATA 203 
effected by means of the tube-feet, or ambulacra. These have 
suckers, which are attached or released by the power of the water- 
vascular system. The madreporic plate, which can be distinctly 
seen in starfishes at the angle of two of the arms, is a caleareous 
porous plate which opens into the stone-canal. This canal passes 
through the body and opens into a circular tube which runs 
around the mouth. <A system of canals extends from this cireum- 
oral tube, or ring-canal, and eventually one canal opens into 
each tube-foot. Water, entering by the madreporic plate, passes 
through the various canals and into the tube-feet, which it dis- 
tends. When the feet are pressed against a substance and the 
“water is withdrawn, a vacuum is formed, which causes the disk- 
ends of the feet to act like suckers. When the feet are again 
flooded the hold is relaxed. Thus the animal drags its body 
along with a slow, gliding motion. The madreporic plate, being 
calcareous, is said to act as a filter, purifying the water as it 
passes into the body, which it furnishes with oxygen as well as 
with locomotive power. 
A curious organ found in the Echinoidea (sea-urchins) is called 
Aristotle's lantern. It is a complicated arrangement, not fully 
understood, having forty parts, and is connected with the alimen- 
tary system. One of the parts is the mouth, 
over which five teeth project. These teeth 
grasp and grind the food. The exoskeleton 
is composed of plates of carbonate of lime, 
called ossicles, which cover the surface of the 
body. The ossicles are of various sizes and 
shapes, and are one of the determining fea- 
tures in classification. They may be scattered 
spicules of lime, separated plates joined by 
connectirfy rods, or overlapping plates; or 
they may be fitted together so as to form a 
continuous shell. The starfishes are usually 
carnivorous, the sea-urchins usually vegetarian, while the holo- 
thurians, after the manner of worms, take in sand and mud, deriv- 
ing their nourishment from the organic particles contained in 
them. All echinoderms live in the sea. 
Aristotle’s lantern. 
