CHAPTER XII. 



Echinodermata. 



THIS name is applied to a sub-kingdom of invertebrate animals which belong 

 entirely to the sea, having no fresh-water or terrestrial representatives. It 

 means " prickly-skinned," and is given to them because the skin is often 

 furnished with prickles or spikes or horny bosses. 



At the present day five kinds occur in our seas, four of which we can find 

 near the shore namely, Sea Urchins, Starfishes, Brittle-stars, and Sea Cucum- 

 bers. The fifth is the Sea Lily, found only in deep water. 



One remarkable feature characterizes all the Echinodermata " everything 

 is governed by the number five." In most cases there are five rays or a 

 multiple of five, the teeth or jaws are five, the boundaries of the plates, 

 and so forth. 



The most familiar creatures belonging to this group are the Starfish, so 

 we will begin with them. 



The Common Five-fingered Starfish (Ur aster rubens). To be found in shallow 

 pools, or cast up on the shore. On the under side of its five rays are the 

 hundreds of "legs" by which it moves. Place a live starfish upside down 

 in some water, and the "legs" which are in reality remarkable suckers, 

 worked by hydraulic power will soon be protruded and be seen madly 

 waving and kicking in all directions. Through a pocket lens these suckers 

 will be found to be tipped with a little cup the sucker proper and the 

 starfish advances by taking hold of the ground with the suckers on its 

 foremost rays and pulling up the hinder ones; then, holding on by their 

 suckers, it pushes forward the other rays, and so moves in a manner not 

 unlike that of a caterpillar. 



As you observe the under side, notice the deep groove along each ray. This 

 is characteristic of the true starfish, the sand-stars and brittle-stars not possess- 

 ing this feature. At the tip of each ray is a red spot called the " eye," which 

 is sensitive to light. The mouth is in the middle of the under side, and opens 

 straight into the stomach, which branches off into each of the rays. 



On the upper side, close to the centre, is a small round stony knob, which is 

 a minute filter, technically known as the " madreporiform plate." Through 

 this the water is taken in to work by pressure the sucker feet, either forcing 

 them out and distending them, or, when the pressure is relaxed, leaving them 



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