126 LIFE BY THE SEASHORE. 



any extent, they possess an extraordinary power of repair- 

 ing accidental injuries. 



Having collected some specimens of the common starfish 

 and placed them in sea-water, the external characteristics 

 can be readily made out. The fact that there are five rays 

 is very obvious, as is also the prickly skin, the ventral 

 mouth, and the five grooves which radiate from the mouth 

 and contain the transparent tube-feet. When the starfish 

 is lifted up from the surface to which it is adhering, it will 

 be noticed that it is attached to this surface by the tube- 

 feet, which end in suckers. So firmly do the suckers cling 

 that it often happens that when the animal is removed from 

 the rock, some of the tube-feet break through rather than 

 let go. When the animal is held in the hand it is easy to 

 feel the limy plates in the skin, and a dried skeletonised 

 specimen picked from the beach will show you the beautiful 

 arrangement of ossicles, or limy plates, which bound the 

 ventral groove along which the tube-feet lie. On the dorsal 

 surface notice between two of the rays a white plate, called 

 the madreporite, or rose-plate, which is perforated by numer- 

 ous holes through which the sea-water enters the system of 

 canals which supplies the tube-feet. These become tense 

 or flaccid according to the amount of fluid they contain, and 

 being alternately fixed and loosened, serve not only for 

 attachment to rock surfaces, but also for leisurely pro- 

 gression. This may occur in any direction, for the starfish 

 being radially symmetrical like a flower, or a sea-anemone, 

 has no head no specialised region which always moves 

 foremost. The radial symmetry (usually based on the num- 

 ber five), the limy skin, the peculiar tube-feet, which are 

 part of the "water-vascular" system, the power of re- 

 generation and frequently of self-mutilation, comprise the 

 most obvious of the external characters of the Echinoderms. 

 There are five living classes : 



1. Starfishes (Asteroids). 



2. Brittle-stars (Ophiuroids). 



3. Sea-urchins (Echinoids). 



4. Sea-lilies (Crinoids). 



5. Sea-cucumbers (Holothurians). 



But of these the sea-lilies only occur in deep water, mostly 



