SEA STABS AND STARFISHES. 49 



and that these shields are pierced with tiny holes called 

 " ambulacral apertures." Through these apertures the 

 ambulacra pass. How they are worked I will try to 

 explain. 



Suppose that a vessel were shaped like an ordinary 

 water carafe, but made of very thin and very elastic 

 india-rubber. Let this vessel be filled with water and 

 the mouth stopped. Now, if the ball be squeezed between 

 the hands, the water will be driven into the neck and 

 cause it to expand. 



Suppose that the neck of the vessel were passed through 

 a hole in a thick log of wood, and were only just large 

 enough to fit ; it could not expand sideways, and must 

 do so downwards, so as to become lengthened. When 

 the pressure is taken off the ball, the water will return 

 into it, and the neck be drawn back through the hole. 



This is just what happens with the starfish, the ambu- 

 lacrum being represented by the carafe, and the pierced 

 log by the ambulacral plate. 



As far as we can see, the starfishes have neither eyes 

 nor organs of scent. Yet they are, in some unknown 

 manner, able to detect the presence of food and to direct 

 themselves towards it. 



Every one who has fished in the sea with a bait of 

 whelk or limpet, must often have been annoyed by find- 

 ing, when they drew in their lines, a starfish fastened on 

 the bait, with its arms clasped tightly round it. Now it 

 is not likely that in these cases the bait has happened 

 to alight in the middle of the starfish, and it is tolerably 

 E 



