CHAPTER XVIII 



ECHINODERMATA {CONTINUED) : ECHINOIDEA = SEA-URCHINS 



CLASS III. ECHINOIDEA 



The Sea-urchins or Echinoidea (Gr. e'^09, Hedgehog or Sea- 

 urchin), which constitute the third class of the Eleutherozoa, 

 have derived both their popular and scientific names from the 

 covering of long spines with which they are provided. At first 

 sight but little resemblance is to be discerned between them 

 and the Starfish and Brittle Stars. They are devoid of any out- 

 growths that could be called arms; their outline is generally either 

 circular or that of an equilateral pentagon, but as their height 

 is almost always smaller than their diameter, they are never 

 quite spherical ; sometimes it is so small that the animals have 

 the form of flattened discs. 



All doubt as to the relationship of the Echinoidea to the Star- 

 fish is at once dispelled in the mind of any one who sees one of the 

 common species alive. The surface is beset with delicate translucent 

 tube-feet, terminated by suckers resembling those of Starfish, 

 although capable of much more extension. The animal throws out 

 these organs, which attach themselves by their suckers to the sub- 

 stratum and so pull the body along, whilst the spines are used to 

 steady it and prevent it from overturning under the unbalanced 

 pull of the tube-feet. When moving quickly the animal walks 

 on its spines, the tube-feet being little used. The tube-feet are 

 distributed over five bands, which run like meridians from one 

 pole of the animal to the other. These bands are termed " radii," 

 and they extend from the mouth, which is situated in the centre 

 of the lower surface, up to the neighbourhood of the aboral pole. 

 The radii must be compared to the ambulacral grooves on the 



503 



