14:2 THE MICROSCOPE AND ITS REVELATIONS. 



of the spine, but (as will presently appear) upon the part of its length 

 from which the section happens to be taken. The centre is usually occu- 

 pied by a very open network (Fig. 367); and this is bounded by a row 

 of transparent spaces (like those at a, a', b b' , c c,' etc., Fig. 369), which 

 on a cursory inspection might be supposed to be void, but are found on 

 examination to be the sections of solid ribs or pillars, which run in the 

 direction of the length of the spines, and form the exterior of every layer. 

 Their solidity becomes very obvious, when we either examine a section 

 of a spine whose substance is pervaded (as often happens) with a coloring 

 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. 369, 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. 



FIG. 369. 



9 

 Portion of transverse section of Spine of Acrocladia mammillata. 



This membrane is probably continued onwards over the whole surface 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 sub- 

 stance. Each new formation completely ensheathes 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 trans- 

 verse 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, notwithstanding that, 

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

 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; form- 

 ing the cup or socket which works over the tubercle of the shell. 



534. Their combination of elegance of pattern with richness of 

 coloring, renders well-prepared specimens of these Spines among the 

 most beautiful objects that the Microscopist can anywhere meet with. 



