CRINOIDS 235 
ten arms in all. The arms have short branches along the sides. 
Ambulacral 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- 
matula, 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 nowremaining 
only twelve of thétwo hundred genera whichexisted 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. 
Pentacrinus asteria. 
