1316 THE STORY OF THE UNIVERSE 



The ridges of the upper surface of a joint fit into 

 the grooves of the lower surface of the joint above 

 it; so that, though from being made up of many 

 joints the stem admits of a certain amount of motion, 

 that motion is very limited. 



As the border of each star-like figure exactly 

 fits the border of the star above and below, the five 

 leaflets within the border are likewise placed directly 

 one above the other. Within these leaflets the 

 limy matter which makes up the great bulk of the 

 joint is more loosely arranged than it is outside, and 

 five oval bands of strong fibres pass in the inter- 

 spaces right through the joints, from joint to joint, 

 from one end of the stem to the other. These 

 fibrous bands give the column great strength. It 

 is by no means easily broken even when dead and 

 dry. They also, by their elasticity, admit a certain 

 amount of passive motion. There are no muscles 

 between the joints of the stem, so that the animal 

 does not appear to be able to move its stalk af 

 will. It is probably only gently waved by the tides 

 and currents, and by the movements of its own arms. 



In Pentacrinus asteria about every seventeenth 

 joint of the lower mature part of the stem is a little 

 deeper or thicker than the others, and bears a whorl 

 of five long tendrils or cirri. These tendrils have 

 no true muscles; they have, however, some power of 

 contracting round resisting objects which they touch, 

 and there are often star-fishes and other sea animals 

 entangled among them. 



Near the head the cirri become shorter and 

 smaller, and their whorls closer. At the top of the 



