478 



OF ECHINODERMATA. 



FIG. 234. 



posed: a a, portions of a deeper layer. 



FIG. 235. 



tion, one to the other; so that in 

 two masses of equal size, the one or 

 the other may greatly predominate ; 

 and the texture may have either a 

 remarkable lightness and porosity, if 

 the network be a very open one, 

 like that of Fig. 235, or may possess 

 a considerable degree of compactness 

 if the solid portion be strengthened. 

 Generally speaking, the different 

 layers of this network, which are 

 connected together by pillars that 

 pass from one to the other in a di- 

 * rection perpendicular to their plane, 

 are so arranged that the perforations 

 in one shall correspond to the inter- 

 mediate solid structure in the next; and their transparency is 

 such, that when we are examining a section thin enough to con- 

 tain 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 ar- 

 rangement, it comes to pass that the 

 plates of which the entire "test" is 

 made up, possess a very considerable 

 degree of strength, notwithstanding 

 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 be rapidly sucked up into its substance. A very beau- 

 tiful example of the same kind of calcareous skeleton, having a 



more regular conformation, is 

 furnished by the disk or ro- 

 sette 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 balsam, the calcareous 

 rosette maybe seen sufficiently 

 well; but its beautiful struc- 

 ture is better made out, when 

 the animal membrane that en- 



One of the segments of the calcareous skeleton of on " miuw ***^ 



Ambulacral disk of Echinus. clOSCS it has DQQU got rid OI 



Transverse Section of the medullary 

 portion of Spine of Acrocladia, showing 

 its more open network. 



FTG. 236. 



