ROSETTE AND SPINE OF ECHINTDA. 



55? 



balsam, the Calcareous Rosette may be seen sufficiently well ; but 

 its beautiful structure is better made-out when the Animal mem- 

 brane that encloses it has been got-rid-of by boiling in a solution 

 of Caustic Potass ; and the appearance of one of the five segments 

 of which it is composed, when thus prepared, is shown in Fig. 288. 



Fig. 



One of the segments of the Calcareous Skeleton of an Ambulacral 

 Disk of Echinus. 



431. The most beautiful display of this Reticulated structure, 

 however, is shown in the structui-e 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 in- 

 termixture, which shows considerable variety in different species. 

 — When we make a thin Transverse Section (Plate n., fig. 1) of 

 almost any Spine belonging to the genus Echinus (the small spines 

 of our British species, however, being exceptional in this respect) 

 or to its immediate allies, we are at once made aware of the ex- 

 istence of a number of Concentric Layers, arranged in a manner 

 that strongly reminds us of the concentric rings of an Exogenous 

 tree (Plate xii., fig. 3). 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. 287) ; and this is bounded by a row of 

 transparent spaces (like those at a a', b V, c c', &c, Fig. 289), which 

 on a cursory inspection might be supposed to be void, 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 



