152 



ECHINODERMA. BLASTOIDEA 



p- 



small plates, the side-plates (s). Extending down the 

 middle of each ambulacrum is the food-groove (a), which, 

 in perfect specimens, is l 



covered over by small 

 plates. At right angles to 

 this 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. Beneath each ambu- 

 lacrum are two hydrospires 

 (fig. 64, h), one on each side. 

 The hydrospire (fig. 65) is a 

 flattened and folded organ, 

 communicating with the exterior by means of the marginal 



Fig 



63. Ambulacrum of Pen- 

 tremites godoni, Carboniferous. 

 I, lancet-plate; s, side-plate; p, 

 pore ; a, food-groove ; sp. spi- 

 racle, x 3. 



Fig. 64. Fig. 65. 



Fig. 64. Pentremites sulcatus, Carboniferous. Horizontal section of the 

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

 (not quite correctly drawn, see fig. 65). (After Zittel.) Enlarged. 



Fig. 65. Pentremites, Carboniferous. Section across ambulacrum, br, 

 brachiole ; cp, covering-plates ; L, lancet-plate ; osp, outer side-plate ; 

 R, radial ; si, sub-lancet-plate ; sp, side-plate. (After Bather), x 5. 



