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 calcareous 

 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 circum- 

 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 water is withdrawn, a vacuum is formed, which causes the disk- 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 Ecninoidea (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 

 connecting rods, or overlapping plates; or 

 they may be fitted together so as to form a 

 continuous shell. The starfishes are usually Annie's lantern. 

 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. 



