268 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
Taking the common Star-fish as our starting- 
point, I have already explained the mechanism 
of its ambulacral system. The animals usually 
crawl in a determinate direction, and when in the 
course of their advance the terminal feet of the 
advancing ray—which are used, not as suckers, but 
as feelers, protruded forwards—happen to come into 
contact with a solid body, the Star-fish may either 
continue its direction of advance unchanged, or 
may turn towards the body which it has touched. 
Thus, for instance, while crawling along the floor ot 
a tank, if the terminal feet at the end of a ray 
_ happen to touch a perpendicular side of the tank, 
the animal may either at once proceed to ascend 
this perpendicular side, or it may continue its 
progress along the floor, feeling the perpendicular 
side with the end of its rays perhaps the whole way 
round the tank, and yet not choosing, as it were, to 
ascend. In the cases where it does ascend and 
reaches the surface of the water, a Star-fish very 
often performs a number of peculiar movements, 
which we may call acrobatic (Fig. 43). On reaching 
the surface, the animal does not wish to leave its 
native element—in fact, cannot do so, because its 
sucking feet can only act under water—and neither 
does it wish again to descend into the levels from 
which it has just ascended. It, therefore, begins to 
feel about for rocks or sea-weeds at the surface, by 
crawling along the side of the tank, and every now 
and then throwing back its uppermost ray or rays 
along the surface of the water to feel for any solid 
support that may be within reach, If it finds one, 
