96 ECHINODERMATA. BLASTOIDEA. 



water-vascular system was placed. On each side of the 

 lancet plate is a row of small plates, the side plates (s). 

 Extending down the middle of each ambulacral area is a 

 groove, the ambulacral groove (a) ; this in some specimens 

 is covered over by a roof of small calcareous plates. At 

 right angles to the ambulacral groove, on each side of it, 

 are numerous transverse grooves. Along the outer margin 

 of the side plates there is a row of pores, the marginal 

 pores (p), formed by spaces between adjoining plates. When 



f 



FIG. 26. Pentremites sulcatus, Carboniferous. Horizontal section of the 

 calyx. I, lancet plate ; p, side plates ; r, radial plates ; hy, hydrospires. 

 (After Zittel.) 



the lancet plate and the side plates are removed from an 

 ambulacral area, we find underneath, in the middle line, 

 an elongated trough-like plate, known as the under lancet 

 plate. At the sides of this may be seen the edges of the 

 hydrospire. The latter consists (fig. 26 A) of a flattened and 

 folded calcareous tube, communicating with the exterior 

 by means of the marginal pores, and also ventrally by the 

 spiracles. A current of water probably passed in through 

 the former openings and out by the latter. In well 

 preserved specimens, the mouth, as in many crinoids, is 



