PHYLUM ECHINODERMATA 



387 



dages, the surface of which is longitudinally fluted. These are 

 movably articulated with the body so that they may be turned 

 about in all directions. When one of them is removed (see Fig. 

 343, p. 413), it is found that the joint is of the character of a ball 

 and socket, a concavity on the base of the spine fitting over a 

 hemispherical elevation on the surface of the Sea-urchin, and the 

 spine being retained in place and caused to move by means of a 

 capsule of muscular fibres enclosing the joint. Around the bases 

 of the large spines are a number of very small spinules. Here and 



:'.2t5. Strongylocentrotus, entire animal with the tube-feet extended. (From 

 Brehm's Tierleben.) 



t-li< re among the spines are to be observed minute pedicellarice 

 Fig. 344, p. 413), which are comparable to the stalked 

 pedicellariae of Asterias ; but each has three jaws instead 

 of two, and a relatively long stalk, which is supported by a 

 slender calcareous rod. Here and there are to be found also small 

 rounded bodies termed the sphceridia, which are perhaps, like the 

 pedicellariae, to be looked upon as modified spines : they contain 

 glion-cells and are apparently organs of special sense, having 

 possibly the function of detecting changes in the composition of 

 the water. 



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