THE ECHINODERMATA. 671 



wouderfully adapted for the purpose to which they are applied, their 

 mouths being supplied with sharp teeth to cut the vessels, and with a 

 sucker-like disc, so that the blood can be drawn from its natural chan- 

 nels ; while their digestive organs are little more than a series of sacs 

 in which an enormous quantity of blood can be received and retained. 



RADIATA. 

 ECHINODERMATA. 



We now arrive at a vast and comprehensive division of living beings, 

 which have no joints whatever and no limbs, and are called Radiata, 

 because all their parts radiate from a common centre. The structure 

 is very evident in some of these beings, but in others the formation is 

 so exceedingly obscure that it is only by anatomical investigation that 

 their real position is discovered. 



The highest forms in this division have been gathered together in the 

 class Echinodermata. This w'ord signifies " urchin-skinned," and is 

 given to the animals comprising it because their skins are more or less 

 furnished with spines resembling those of the hedgehog. In these 

 animals the radiate form is very plainly shown, some of them assuming 

 a perfectly star-like shape, of which the common star-fishes of our 

 coasts are familiar examples. In some of the Radiates, such as the 

 sea-urchin, the whole body is encrusted with a chalky coat, while in 

 others it is as soft and easily torn as if it were composed of mere 

 structureless gelatine. 



The mode of walking, or rather creeping, which is practised by these 

 beings is very interesting, and may easily be seen by watching the pro- 

 ceedings of a common star-fish when placed in a vessel of sea-water. 

 At first it will be quite still and lie as if dead, but by degrees the tips 

 of the arms will be seen to curve slightly, and then the creature slides 

 forward without any perceptible means of locomotion. If, however, it 

 be suddenly taken from the water and reversed, the mystery is at once 

 solved, and the walking appartus is seen to consist of a vast number of 

 tiny tentacles, each with a little round transparent head, and all mov- 

 ing slowly but continually from side to side, sometimes being thrust out 

 to a considerable distance, and sometimes being withdrawn almost wholly 

 within the shell. These are the " ambulacrie," or walking apparatus, 

 and are among the most extraordinary means of progression in the an- 

 imal kingdom. Each of these innumerable organs acts as a sucker, its 

 soft head being applied to any hard substance, and adhering thereto 

 with tolerable firmness until the pressure is relaxed and the sucker re- 

 leased. The suckers continually move forward, seize upon the ground, 

 draw the body gently along, and then search for a new hold. As there 



