OR OSSEOUS SYSTEM. 17 



FIG. 7. 



of the rays, and form the ambulacral grooves below for the 

 tubular fleshy feet. Besides the tuberculated coriaceous 

 irritable skin covering the upper or dorsal part of each 

 small segment of each ray, we can generally distinguish eight 

 calcareous plates placed transversely on each segment, and 

 surrounding its sides and lower surface. In the sea-star 

 represented in the figure there are eighty of these transverse 

 divisions or segments in each of the five rays of the animal. 

 In all the segments of a single ray there are about seven 

 hundred plates, and about three thousand five hundred cal- 

 careous pieces in the segments of the whole animal. The 

 concave lower surface of each ray is perforated by numerous 

 pairs of small oblique holes placed on each side of a longitu- 

 dinal median line, which are the ambulacral perforations for 

 the tubular fleshy suckers, by which these animals drag 

 themselves along the bottom of the sea, or up the perpendi- 

 cular sides of rocks. The lateral and dorsal parts of the 

 segments often support fixed or moveable spines which grow 

 like the plates themselves by the successive addition of 

 calcareous layers from the thin fleshy secreting membrane 

 which covers every part of the calcareous skeletons ofechino- 

 derma. 



On the back of these animals, and a little to one side of 

 the centre, between the commencement of two of the rays, 

 there is a small, round, solid, calcareous body, represented 

 at by in Fig. 7? A. This round calcareous plate is convex 



PART I. C 



