STKUCTURE OF ECHINOLDS 885 



lar to their plane, are so arranged that the perforations in one shall 

 correspond to the intermediate solid structure in the next ; and their 

 transparence is such that when we are examining a section thin 

 enough to contain only two or three such layers, it is easy, by 

 properly focussing the microscope, to bring either one of them into 

 distinct view. From this very simple but very beautiful arrange- 

 ment, it comes to pass that the plates of which the entire * test ' is 

 made up possess a very considerable degree of strength, notwith- 

 standing that their porousness is such that if a portion of a fractured 

 edge, or any other part from which the investing membrane has 

 been removed, be laid upon fluid of almost any description, this will 

 l>e rapidly sucked up into its substance. A very beautiful example 

 of the same kind of calcareous skeleton, having a more regular con- 

 formation, is furnished by the disc or * rosette ' which is contained 

 in. the tip of every one of the tubular suckers put forth by the living 

 Echinus from the ' ambulacral pores ' that are seen in the rows of 



FIG. 671. Section of shell of Echinus FIG. 672. Transverse section of cen- 



showing the calcareous network of tral portion of spine of Heterocen- 



which it is composed : a a, portions trotus, showing its more open net- 



of a deeper layer. work. 



smaller plates interposed between the larger spine-bearing plates of 

 its box-like shell. If the entire disc be cut off, and be mounted 

 when dry in Canada balsam, the calcareous rosette may be seen 

 sufficiently w r ell ; but its beautiful structure is better made out when 

 the animal membrane that incloses 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. 674. 



The most beautiful display of this reticulated struct ure, however, 

 is shown in the conformation of the l 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) or its immediate allies, we see it to be 



