558 



STRUCTURE OF SPINES OF ECHIN1DA. 



form the exterior of every layer. Their solidity becomes very ob- 

 vious, when we either examine a section of a Spine whose substance 

 is pervaded (as often happens) with a Colouring matter of some 

 depth, or when we look at a very thin section by the 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. 289, the outermost 



Fig. 289. 



Portion of transverse section of Spine of Acrocladia mammillata. 



row of pillars forming the projecting ribs that are very 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 which 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. 

 This membrane is probably continued onwards over the whole sur- 

 face of the Spine, although it cannot be clearly traced to any 

 distance from the base ; and the new formations may be presumed 

 to take-place in its substance. Each new formation completely 

 ensheaths the old ; not merely surrounding the part previously 

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

 happens that the number of Layers shown in a Transverse Section 

 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, notwithstand- 

 ing that, in the club-shaped Spines, this terminal portion may be 

 of considerably larger diameter than the basal ; and in any inter- 

 mediate part of the Spine, so many layers will be traversed as have 

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



