CHAP, v ECHINODERMA TA 61 



which prevents it from being confused with the smaller, stiff 

 starfish, Henricia, which is almost equally common. 



All these Starfishes live on the sea-floor, 



DrsaTsurface craw ^ n g oyer the roc ^ s an d sand amongst the 



seaweed. In essential structure all starfishes 



are very similar. The body consists of a central disc-like 



portion with five radiating arms, which are all alike both in 



FIG. 30. Asterias rubens (life-size). 



Dorsal view : one arm is raised to show the tube-feet, if, on the ventral surface ; 

 md, madreporite. 



external and internal structure, and all of which are equally 

 capable of moving forwards and dragging the rest of the 

 body after them. The surface of the body feels rough to the 

 touch, for in the body- wall are embedded very numerous small 

 calcareous plates or "ossicles," which protect the body and 

 yet do not impede its motion, as they can move freely one on 

 another. Many of these plates bear small simple spines, and 

 amongst these may be seen small pincer-like bodies mounted 

 on short stalks ; these are known as " pedicellariae " (Fig. 



