STAR-FISH AND SEA-URCHINS. 275 
or spiral travels progressing all the way down the 
ray. Usually two or three adjacent rays perform 
this manceuvre simultaneously; but if, as some- 
times happens, two opposite rays should begin to 
do so, one of them soon ceases to continue the 
manceuvre, and one or both of the rays 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 of 
the active rays. The whole movement does not 
occupy more than half a minute. As a general 
rule, the rays are from the first co-ordinated to 
effect the righting movement in the direction in 
which it is finally to take place—the rays which 
are to be the active ones alone twisting 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, rights itself after the manner shown in Fig. 48, 
where the animal is represented as standing on the 
tips of four of its rays, while the fifth one is just 
about to be thrown upwards and over the others, in 
order to carry with it the two adjacent rays, and so 
