556 



CALCAREOUS SKELETON OF ECHTNODERMATA. 



Section of Shell of Echinus, showing the 

 calcareous network of which it is com- 

 posed :— a a, portions of a deeper layer. 



predominate ; and the texture may have either a remarkable light- 

 ness and porosity, if the network be a very open one like that of 



Fig. 287, or may possess 

 Fig. 286. a considerable degreee of 



compactness, if the solid 

 portion be strengthened. 

 Generally speaking, the 

 different layers of this 

 Network, which are con- 

 nected together by pillars 

 that pass from one to 

 the other in a direction 

 perpendicular '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 arrangement, it comes to pass that the plates of 



which the entire ' Test ' 

 Fig. 287. is made-up possess a very 



considerable degree of 

 strength, notwithstand- 

 ing that their porous- 

 ness is such that if a 

 portion of a fractured 

 edge, or any other part 

 from which the invest- 

 ing membrane has been 

 removed, be laid upon 

 fluid of almost any de- 

 scription, this will be 

 rapidly sucked-up into 

 its substance. — A very 

 beautiful example of the 

 same kind of Calcareous 

 Skeleton, having a more 

 regular conformation, is furnished by the Disk 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 of its shell. If 

 the entire Disk be cut-off, and be mounted when dry in Canada 



Transverse Section of central portion of 

 Spine of Acrocladia, showing its more open 

 network. 



