ECHINODERMATA. 



141 



able 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 

 beautiful example of the same kind of calcareous skeleton, having a more 

 regular conformation, is furnished by the disk or ' rosette ' which is con- 

 tained in the tip of every one of the tubular suckers put forth by the 



FIG. 367. 



Section of Shell of Echinus, showing 

 the calcareous network of which it is 

 composed: a a, portions of a deeper 

 layer. 



Transverse Section of central portion 

 of Spine of Acrocladia, showing its more 

 open network. 



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

 smaller plates interposed between the larger spine-bearing plates of its 

 box-like shell. If the entire disk be cut-off, and be mounted when dry 

 in Canada balsam, the calcareous rosette may be seen sufficiently well; 

 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. 368. 

 533. The most beautiful display 



f, * , . -, , -, -,* rlG. 000. 



this reticulated structure how- 

 ever, is shown in the structure of 

 the 'spines' of Echinus, Cidaris, 

 etc. ; 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 ela- 

 borate pattern being formed by 

 their intermixture, 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 its immediate allies, we see it to be made up of a number of 

 concentric layers, arranged in a manner that strongly reminds us of 

 the concentric rings of an Exogenous tree (Fig. 254). The number of 

 these layers is extremely variable; depending not merely upon the age 



One of the segments of the calcareous skeleton of 

 an Ambulacral Disk of Echinus. 



