CRINOIDS 



235 



ten arms in all. The arms have short branches along the sides. 

 Arnbulacral grooves follow the center of all the arms and 

 branches. The mouth is in the 

 center of the upper side, which 

 in this case is the ventral surface. 

 The genus Pentacrinus remains 

 permanently attached, but Co- 

 ni a tula, at a certain stage of de- 

 velopment, separates from the 

 stalk and swims freely about by 

 means of its arms. It can attach 

 itself temporarily by tubular pro- 

 cesses, which are developed on the 

 dorsal surface at the point where 

 the stem was attached. The ossi- 

 cles, or plates which cover the 

 dorsal surface, are free, making 

 the crinoid an animal of innu- 

 merable joints. 



The crinoids are particularly 

 interesting from the fact that they 

 have existed from early geologic 

 times, and their history is written 

 in stone. In the early ages they 

 were the only class of echino- 

 derms, and their evolution into 

 other forms can be traced through 

 successive geologic periods. They 

 existed in such vast numbers that 

 the fossil forms are plentiful and 

 are familiar to every student of 

 geology, and are known as stone- 

 lilies and encrinites. To-day they 

 are decadent, there now remaining 

 only twelve of the two hundred genera which existed formerly. A fine 

 bed of crinoids is found off Cuba, on the slope of the coast where the 

 water rapidly deepens from one hundred to two hundred fathoms. 



Pentacrw/us asteria. 



