SPINES OF ECHINUS, CTDABIS, AND SPATANGUS. 559 



portion of the Spine is enveloped in a reticulation of a very close 

 texture, without concentric layers ; forming the Cup or socket 

 which works over the Tubercle of the Shell. 



432. The combination of elegance of Pattern with richness of 

 Colouring renders well-prepared specimens of these Spines among 

 the most beautiful objects that the Microscopist can anywhere 

 meet-with. The large spines of the various species of the Genus 

 Acrodaclia furnish sections most remarkable for size and elabo- 

 rateness, as well as for depth of colour (in which last point, how- 

 ever, the deep purple spines of Echinus lividus are pre-eminent) ; 

 but for exquisite neatness of pattern, there are no spines that can 

 approach those of Echinometra heteropora (Plate n., fig. 1) and 

 E. lucunter. The spines of Heliocidaris variolaris are also re- 

 markable for their beauty. — No succession of concentric layers is 

 seen in the spines of the British Echini, probably because (accord- 

 ing to the opinion of the late Sir J. Gr. Dalyell) these Spines are 

 cast-off and renewed every year ; each new formation thus going 

 to make an entire Spine, instead of making an addition to that pre- 

 viously existing. — Most curious indications are sometimes afforded 

 by Sections of Echinus -spines, of an extraordinary power of Repa- 

 ration inherent in these bodies. For irregularities are often seen 

 in the transverse sections, which can be accounted-for in no other 

 way than by supposing the Spines to have received an injury when 

 the irregular part was at the exterior, and to have had its loss of 

 substance supplied by the growth of new tissue, over which the 

 subsequent layers have been formed as usual. And sometimes a 

 peculiar ring may be seen upon the surface of a spine, which in- 

 dicates the place of a complete fracture, all beyond it being a new 

 growth, whose unconformableness to the older or basal portion is 

 clearly shown by a longitudinal section. — 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 encased in a solid calcareous Sheath 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 dia- 

 meter, save on the outside of the 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 distinguished from those of Echini. — The 

 slender, almost filamentary Spines of Spatangus (Fig. 290), and 

 the innumerable minute Hair- like processes attached to the shell 

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

 stance ; and these are very beautiful objects for the lower powers of 

 the Microscope, when laid upon a Black Ground and examined by 

 reflected light without any further preparation. — It is interesting 



