CALCAEEOUS SKELETON OF ECHINIDA. 



479 



by boiling in caustic potass ; and tbe appearance of one of the 

 five segments of which it is composed, when thus prepared, is 

 shown in Fig. 236. 



313. The most beautiful display of this reticulated structure, 

 however, is shown in the structure of the "spines" of Echinus, 

 Cidaris, &c. ; in which it is combined with solid ribs or pillars, 

 disposed in such a manner as to increase the strength of these 

 organs ; a regular and elaborate pattern being formed by their 

 intermixture, which shows considerable variety in different 

 species. "When we make a thin transverse section of almost any 

 spine belonging to the genus Echinus (the small spines of our 

 British species, however, being exceptional in this respect), we 

 are at once made aw^are of the existence of a number of concen- 

 tric layers, arranged in a manner that strongly reminds us of the 

 concentric rings of an Exogenous tree (Fig. 167). The number 

 of these layers is extremely variable ; depending not merely 

 upon the age 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 occupied by a very open network 

 (Fig. 235) ; and this is bounded by a row of transparent spaces 

 (like those at a a', h V, c c'. Fig. 237), which, on a cursory inspec- 

 tion, might be supposed to be void spaces, but which on a closer 

 examination are found to be the sections of solid ribs or pillars, 

 which run in the direction of the length of the spine, and form 

 the exterior of every layer. Their solidity becomes very obvi- 

 ous, when we either examine a section of a spine whose substance 

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



Fig. 237. 



/ g 



Portion of transverse secliou of Spine of A^rodadUi mammillata. 



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. 

 237, the outermost 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 



