128 THE STORY OF ANIMAL LIFE 



sucking disc of each catches hold of the ground. 

 Then the feet are again retracted, while their 

 discs still cling; the effect of this is, naturally, to 

 pull the ray onwards. This process is repeated 

 again and again, until some appreciable degree of 

 movement is effected. The tube-feet are in con- 

 nection with a system of vessels filled with fluid, 

 known as the Water-vascular System of the Star- 

 fish. The fluid is driven on by muscular contrac- 

 tions until the feet are fully extended, and again 

 driven back when the feet are retracted. The 

 Water-vascular System is a structure common to 

 all Echinoderms ; and vessels of a comparable 

 character are found in some worms. 



How does the Starfish know where it is going? 

 Underneath each ray, near the tip, is a little feeler 

 (or tentacle) and a little eye spot. By means of 

 these it gets an idea where each ray is going to ; 

 and, since it often moves but one ray at a time, 

 this is sufficient for it. When necessary, how- 

 ever, the several rays can act in concert with one 

 another. 



The rayed form of the Starfishes led to their 

 being at first included in the group of Radiate 

 Animals, along with the tentacle-bearing Ccelen- 

 terata; but it has long been recognised that they 

 are animals of much higher structure. Their very 

 larvae can barely be brought into comparison with 

 animals so simple as the true " radiates." 



The Snake-Stars, or Ophiuroidea, are closely 

 allied to the Starfishes. In these the arms are 

 thin and sharply defined from the little central 

 disc, instead of sloping gently out of it, as in the 

 Starfishes. The rapid wriggling movements of 

 the arms have gained for them their very appro- 

 priate name. They are also called Brittle Stars, 



