104 



MARINE ANIMALS OF MASSACHUSETTS BAY. 



direction, fastens them by means of the suckers on some surface, 

 be it of rock, or shell, or the side of the glass jar in which they 

 are kept, and being thus anchored it drags itself forward. The 

 tentacles are of a violet hue, though when stretched to their 



Fig. 135. 



greatest length they lose their color, and become almost white 

 and transparent ; but in their ordinary condition the color is 

 quite decided, and the rows along which they occur make as 

 many violet lines upon the surface of the body. 



Almost the sole function of the spines seems to be that of pro- 

 tecting the animal, and enabling it to resist the attacks of its ene- 

 mies, the force of the waves, or any sudden violent contact with 

 the rocks. The spines, when magnified, are seen to be finely ribbed 

 for nearly the whole length (Fig. 135), the bare basal knob serv- 

 ing as the point of attachment for the powerful muscles, which 

 move these spines on a regular ball-and-socket joint, the ball sur- 

 mounting the tubercles (seen in Fig. 132), which fit exactly in a 

 socket at the base of the spine. In a transverse section of a spine 

 (Fig. 136), we see that the ribs visible on the outside are delicate 



Fijr. 134. Sea-urchin seen from the mouth side. (Agassiz.) 

 Fig. 135. Magnified spine. 



