ENCRINITES. 113 



they are furrowed on the upper surface. Both the 

 arms and the pinnae which give to them such a 

 feathered appearance are formed of an immense 

 number of limy joints. (In the extinct Pentacrinus 

 briareus, found so abundantly in the Lias near 

 Whitby, it is estimated that no fewer than one hun- 

 dred and fifty thousand joints are employed in the 

 construction of the five pinnated arms of one 

 individual !) All are alike grooved on their upper 

 surface, and thus we have channels or gutters running 

 over every part of the upper surface of each arm. All 

 this was covered, when the Encrinite was alive, by 

 membrane and muscles, which kept the ossicles 

 together. The membrane was covered with thou- 

 sands of minute cilia. The latter were movable, and 

 formed a motive machinery much in use among all 

 kinds of the lower forms of aquatic animal life. The 

 consequence of the general action of these vibratile 

 cilia over the entire upper surface of the arms of the 

 Encrinites was, that currents of water bearing food 

 were constantly being deflected down the bases of the 

 five arms. The main grooves of these were continued 

 over the surface of the body of the Encrinite, and all 

 converged towards the mouth, which was thus sup- 

 plied with fresh food and fresh water. 



In the Palaeozoic Crinoids the arms are grooved 

 above, but the grooves terminate at their bases, and 

 do not continue over the surface of the body as 

 just described. Instead they open into tunnels or 



I 



