18 ORGANS OF SUPPORT, 



above, and presents an exterior surface marked by numerous 

 tortuous grooves, like the surface of a meandrina. In its 

 concave interior surface it protects a small membranous sac, 

 which contains a thick grumous matter chiefly composed of 

 carbonate of lime with a little phosphate, and was supposed 

 by Tiedemann to be the organ which separates from the fluids 

 of the body the calcareous matter of the exterior covering. 

 The arrangement of the plates enveloping the segments is 

 very similar to this in all the other forms of asterias, how- 

 ever they may vary in the number of the rays, and in the 

 ophiurce where the rays do not contain prolongations of the 

 digestive and generative organs,, and in the other forms of 

 stellerida. 



In the more compact forms of the echinida, the skeleton 

 is more solid, contains more phosphate of lime, and the 

 component plates are arranged with more obvious symmetry. 

 The plates are arranged in perpendicular or longitudinal 

 columns extending from around the mouth to the anus, as is 

 seen in the figure of the common echinus esculentus, (Fig. 7 5 

 B.) which represents the entire shell, as seen from above, 

 and deprived of its exterior spines. Some of these vertical 

 columns are seen to be perforated with the same kind of 

 small round oblique ambulacral holes as in the asterias. 

 These perforated ambulacral columns, (Fig. 7? B, ,,,) are ten 

 in number, disposed two and two together, so as to form five 

 pairs. The letters a, a, a, point to the middle line of separation 

 between each pair of ambulacral columns. Between these 

 five pairs of small perforated columns are placed alternately five 

 pairs of columns of. larger tuberculated plates which are not 

 perforated for the feet. These tuberculated columns in pairs 

 interposed between the successive pairs .of ambulacral 

 columns are seen in Fig. 7 5 B at b ; and the line of separa- 

 tion between the tuberculated and the ambulacral columns 

 is represented at c, c. The whole exterior of the shell being 

 covered in the recent state with moveable spines attached to 

 the tubercles we cannot perceive the arrangement of these 

 vertical columns of separate plates till the spines are re- 

 moved, or the shell is broken and viewed on the inner sur- 

 face. 



A small portion of the shell of the echinus esculentus, 

 magnified and viewed from the inner surface, where the ar- 



