SPINES OF ECHlNOIDS 887 



either examine a section of a spine whose substance is pervaded (MS 

 often happens) with a colouring matter of some depth, or when we 

 look at a very thin section by black-ground illumination. Around 

 the innermost circle of these solid pillars there is another layer of 

 the calcareous network, which again is surrounded by another circle 

 of solid pillars ; and this arrangement may be repeated many times, 

 as shown in fig. 675, the outermost row of pillars forming the 

 projecting ribs that are commonly to be distinguished on the surface 

 of the spine. Around the cup-shaped base of the spine is a membrane 

 which is continuous with that covering the surface of the shell, and 

 serves not merely to hold down the cup upon the tubercle over which 

 it works, but also by its contractility to move the spine in any required 

 direction. The increase in size of the spine appears to be due to the 

 protoplasmic substance which fills up the spaces in the open network 

 of the spine and other skeletal structures. Each new formation 

 completely ensheathes the old, not merely surrounding the part pre- 

 viously formed, but also projecting considerably beyond it ; and thus 

 it happens that the number of layers shown in a transverse section 



FIG. 675. Portion of transverse section of spine of Heterocentrotua 

 mammillatus. 



will depend in part upon the place of that section. For if it cross 

 near the base, it will traverse every one of the successive layers from 

 the very commencement ; whilst if it cross near the apex, it will 

 traverse only the single layer of the last growth, notwithstanding 

 that, in the club-shaped spines, this terminal portion may be of con- 

 siderably larger diameter than the basal ; and in any intermediate 

 part of the spine, so many layers will be traversed as have been 

 formed since the spine first attained that length. The basal 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. 



Their combination of elegance of pattern with richness of colour- 

 ing 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 Heterocentrotus 

 furnish sections most remarkable for size and elaborateness, as well 

 as for depth of colour (in which last point, however, the deep purple 

 spines of Echinus lividus are pre-eminent) ; but for exquisite 



