ECHINODERMATA — CRINOIDS 1 59 



connected with an oral ring. The anal opening is fused with 

 the largest spiracle. 



The calyx has thirteen definitely arranged calcareous plates, 

 — three basals, five large incised radials, and five interradials 

 or deltoids. The plates are firmly united. The structure of 

 the stem is similar to that of the crinoids. 



Pentremites was exceedingly abundant in the sea which 

 covered most of North America during Mississippian time. 



1. Sketch (a) side view, (b) ventral view. Label calyx, 

 mouth, spiracles, ambulacra, marginal pores, anal opening, 

 place for attachment of stalk. 



2. Explain the process by which the animal ate. 



3. How did it probably respire ? 



4. Compare it to a starfish. 



5. Give the geologic range of Pentremites. 



6. What was its habitat ? 



7. What part of the living animal does the fossil represent? 



8. Could the animal move ? Reasons. 



CLASS C, CRINOIDEA (SEA-LILIES) 



Body, the calyx, more or less globidar, fastened, tem- 

 porarily or permanently, to some foreign object by means of a 

 stalk arising from its dorsal (lower) surface. The ventral 

 (upper) surface bears the mouth, which is usually central, the 

 anus, which is excentric, and the ambulacral grooves. These 

 grooves extend into the more or less complexly branched arms, 

 which are usually bordered with pinnules. Some forms, as 

 Eucalyptocrinus, were attached by roots, some, as Pentacrinus 

 and Ancyrocrinus, anchored themselves to seaweeds and other 

 objects by processes from the lower end of the stalk, and some 

 probably used their stems for coiling around a support. 



Crinoids range from the Ordovician to the present time, 

 though they were most abundant in the upper Paleozoic. Owing 

 to their gregarious habits, their skeletons form the chief part of 

 great masses of limestone (hence called crinoidal limestone), 

 especially in the Devonian and Mississippian periods. 



