ECHINODERMA. BLASTOIDEA 



153 



pores, and also by the spiracles on the oral surface of the 

 calyx. 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 not visible 

 externally, but is covered over by a roof of small plates. 

 From the margins of the ambulacra pinnule-like append- 

 ages known as brachioles (fig. 61) are given off; these 

 are seldom preserved, but pits or facets to which they 

 were attached are seen on the side-plates. 



The hydrospires are really folded parts of the radial 

 and deltoid plates — the folds being parallel to the margin 

 of the ambulacra. This is seen clearly in Codaster, which 

 is a more primitive form than Pentremites ; in that genus 

 (fig. 66) the folds open directly to the exterior by slits, 



br. 

 c.p. s.p^y R.pr. 



rOSPL 



Fig. 66. Fig. 67. 



Fig. 66. Codaster trilobatus, Carboniferous. Section across ambulacrum. 

 (After Bather.) x 5. 



Fig. 67. Phcenoschisma verneuili, Carboniferous. Section across ambu- 

 lacrum. (After Bather.) Enlarged. 



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

 plate ; R, radial ; Rpr, part of radial ; sp, side-plate. 



owing to the fact that they are not covered by the lancet- 

 plate and side-plates ; and on account of this circumstance 

 spiracles are not developed. In some genera, in which 

 the folds are concealed (fig. 67), the space below the 

 lancet-plate and side-plates, into which the folds open, 

 communicates with the exterior at the oral end by slits 



