STAR-FISH AND SEA-URCHINS. 275 



or spiral travels ])rogressing all the way down the 

 ra}'. Usually two or three adjacent rays perform 

 this mantieuvre simultaneously ; but if, as some- 

 times ha])pens, two opposite rays should begin to 

 do so, one of them soon ceases to continue the 

 manoeuvre, and one or both of the ra3^s adjacent to 

 the other takes it up instead, so assisting and 

 not thwarting the action. The spirals of the co- 

 operating rays being invariably turned in the same 

 direction (Fig. 47, a, b, and c), the result is, when 

 they have proceeded sufficiently far down the rays, 

 to drag over the remaining rays, which then 

 abandon their hold of the bottom of the tank, so as 

 not to offer any resistance to the lifting action oi 

 the active rays. The whole movement does not 

 occupy moie than half a minute. As a general 

 rule, the rays are from the first co-ordinated to 

 effect the rIo*htin2f movement in the direction in 

 which it is finally to take place — the ra3^s which 

 are to be the active ones alone twistinof over, and 

 so twisting that all their spirals turn in the same 

 direction. 



A Star-fish (Astropecten) which is intermediate 

 between the Brittle-star and the common Star-fish, in 

 that its ambulacral feet are partly aborted (having 

 lost their suckers, as shown in Fig. 44) and its 

 rays more mobile than those of the common Star- 

 fish, rio'hts itself after the manner shown in Fi^;. 48, 

 where the animal is represented as standing on the 

 tips of four of its rays, while the fifth one is just 

 about to bo thrown upwards and over the others, in 

 order to cai ry with it the two adjacent rays, and so 



