SPINES OF SPATANGUS. 481 



314. The spines of Cidaris present a marked departure from 

 the plan of structure exhibited in Echinus ; for not only are they 

 destitute of concentric layers, but the calcareous network which 

 forms their principal substance, is ensheathed in a solid cal- 

 careous cylinder perforated with tubules, which seems to take 

 the place of the separate pillars of the Echini. This is usually 

 found to close in the spine at its tip also ; and thus it would 

 appear that the entire spine must be formed at once, since no 

 addition could be made either to its length or to its diameter, 

 save on the outside of this sheath, where it is never to be found. 

 The sheath itself often rises up in prominent points or ridges on 

 the surface of these spines; thus giving them a character by 

 which they may be distinguishe'd from those of Echini. The 

 slender, almost filamentary spines of Spatangus (Fig. 238), and 

 the innumerable minute hair-like processes attached to the shell 

 of Olypeaster, are composed of the like regularly reticulated sub- 



FlG. 238. 



Spines of Spatangus. 



stance ; and many of these are very beautiful objects for the 

 lower powers of the Microscope, when examined by reflected 

 light, and laid upon a black ground, without any further pre- 

 paration. It is interesting also to find that the same structure 

 presents itself in the curious Pedicellarice (forceps-like bodies 

 mounted on long stalks), which are found on the surface of 

 many Echinida, and the nature of which has been a source of 

 much perplexity to l^aturalists, some maintaining that they are 

 parasites, whilst others consider them as proper appendages of 

 the Echinus itself. The complete conformity which exists be- 

 tween the structure of their skeleton and that of the animal to 

 which they are attached, would seem to remove all reasonable 

 doubt of their being truly appendages to it, as obsei-vation of 

 their actions in the living state would indicate. Another ex- 

 atnple of the same structure is found in the peculiar system of 

 plates which surrounds the interior of the oral orifice of the 

 shell, and which gives support to the five teeth that may often 

 be seen projecting externally through that orifice; the whole 

 forming what is known as the "lantern of Aristotle." The 



31 



