158 AN INTRODUCTION TO THE STUDY OF FOSSILS 



a food, or ambulacral groove, and from this groove branch off 

 smaller side grooves almost at right angles, the whole forming a 

 V-shaped ambulacral area. 



At the upper end of each side groove is a small pore (marginal 

 pore) ; these pores open upon the inside of the calyx into a cal- 



IV 



Fig. 61. — This blastoid, Pentremites pyrijormis Say, was very prolific in the clearer 

 portions of the sea {i.e. Kentucky, Alabama, and Mississippi), which covered much 

 of North America during Kaskaskia (Upper Mississippian) time. A, side view 

 (natural size) with stem missing, as is nearly always the case. B, top view. C, 

 cross section ( X 2) at a-b in Fig. A. amb., ambulacrum; an., anal opening fused 

 with a spiracle ; ba., basal plate ; f.gr., food groove; hy., hydrospires (to conduct 

 water from the marginal pores out through the spiracles) ; i.r., interradial plates; 

 w./>o., marginal pores ; wo., mouth; .?/)., spiracles ; r., radial plates ; j/., place 

 of attachment of stem. 



careous canal looking like a much-folded bag, — the hydrospire, 

 probably respiratory in function. Each of the five pairs of 

 hydrospires from adjoining ambulacra end in one of the spiracles. 

 During life, it is probable that a current of water passed in at 

 the marginal pores, through the hydrospires, and out at the 

 spiracles. Around the marginal pores were long, jointed pin- 

 nules which functioned in straining out the food particles ; this 

 food, probably similar to that of crinoids, was urged down the 

 side grooves by the cilia lining them, and thus down the central 

 groove to the mouth. In exceptionally well-preserved speci- 

 mens the central and side food grooves to the base of the pinnules 

 are roofed over by very minute plates. The position of the 

 pinnules is indicated by minute pits and knobs. 



Little is known concerning the digestive, blood vascular, and 

 nervous systems ; there is some evidence of radial nerve cords 

 connected with an oral ring, and of radial blood vessels likewise 



