ECHINODERMATA. 



227 



rous, surrounding the aperture of the mouth, situated in the 

 centre of a plaited integument which extends over the abdo- 

 minal cavity, and is capable of being contracted into a conic 

 or proboscal shape." 



The Encrinus moniliformis (fig. 159) from the muschelkalk, 

 and the Apiocrinites rotundatus (fig. 160) from the Bradford 

 clay, will convey an idea of the structure of this interesting 

 order. 



In the most perfect type of crinoidean the skeleton is 

 composed of three distinct parts. The root (fig. 160), which 

 fixes the animal permanently by a calcareous secretion to 

 some sub-marine body: its form varies in the different 

 genera. The stem, more or less elongated, is composed of 

 calcareous pieces, piled on each other to form a flexible 

 column for the support of the body and arms. The indivi- 

 dual pieces are of a circular or pentagonal form. Their 

 articulating surfaces are exquisitely sculptured of various 



FIG. 159. Encrinites moniliformis. 



FIG. 160. Apiocrinites rotundus. 



figures which are permanent in the species, and afford 

 characters for identifying the same. The stem in apio- 

 crinites is cylindrical, slender in the middle part and enlarging 



Q2 



