294 



ECHINODERMS. 



at some time of their lives attached by a stalk to the sea-floor, or some other object, 

 so that the mouth and vent naturally move up to that side of the body furthest 

 from the stalk. This fixed state of existence has also caused the development of 

 arms, five in number, but often forked many times, which arms stretch out from 

 the body on all sides of the mouth, and contain extensions of the nervous, blood- 

 vascular, water-vascular, and generative systems. The representatives of the tube- 

 feet are arranged along the sides of these arms, on their upper or oral surface, and 

 between them is a groove, which is lined at the bottom with cilia, or extremely 

 minute hair-like processes, that keep waving in the direction of the mouth, and so 



GROUP OF STONE-LILIES (PENTACMNIDS). 



maintain a constant stream of water towards the latter ; such water containing the 

 minute animalculae and fragments of decaying organic matter on which the crinoid 

 feeds. The extinct cystids and blastoids have their mouth in a similar position to 

 that of the crinoids, and for a similar reason, but have not similarly branched 

 arms. In the blastoids five grooves radiate down the body from the central 

 mouth, and from the sides of these grooves there spring small, jointed, but 

 unbranched processes, called pinnules. The stem of the blastoids is very short, 

 so that when the pinnules have been lost, as is usually the case, the five-grooved 

 body looks like a bud, whence the name of the class. It is difficult to describe a 

 cystid as having any definite shape, for the various animals to which this name is 

 applied differ greatly from one another in structure. Echinoderms are built upon 



