26S JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



Takinor the common Star-fish as our stavtingr- 

 point, I have ahxady 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 bod}^ 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 ascendcJ. It, therefore, begins to 

 feel about for rocks or sea-weeds at the surface, by 

 crawlinof alon^' 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, 



