ECHLNODERMATA. 143 



The large spines of the various species of the genus Acrocladia furnish 

 sections most remarkable for size and elaborateness, as well as for depth 

 of color (in which last point, however, 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 II., 

 fig. 1) and E. lucunter. The spines of Heliocidaris variolaris are also 

 remarkable for their beauty. No succession of concentric layers is seen 

 in the spines of the British Echini, probably because (according to the 

 opinion of the late Sir J. G. 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 previously existing. Most 

 curious indications are sometimes afforded by sections of Echinus-spines, 

 of an extraordinary power of reparation 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 



FIG. 370. 



Spines of Spatangus. 



a peculiar ring may oe seen upon the surface of a spine, which indicates 

 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. 1 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 

 diameter, 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. 370), and the innumerable minute 



'See the Author's description of such Reparations in the "Monthly Micro- 

 scopical Journal," Vol. iii. (1870), p. 225. , 



