GENERAL CHARACTERISTICS 



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far as we know, all crinoids are 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 exten- 

 sions 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 direc- 



GROUP OF STONE LIWES (PENTACRINIDS) . 



tion of the mouth, and so maintain a constant .stream of water toward 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 



