SEA-URCHINS AND SEA-CUCUMBERS. 279 



numerous polygonal plates joined to each other with great 

 exactness, and beset on its external surface with * spines,' 

 which may have the form of prickles of no great length, 

 or may be stout, club-shaped bodies, or, again, may be 

 very long and slender rods. The intimate structure of the 

 shell is everywhere the same : for it is composed of a net- 

 work, which consists of carbonate of lime, with a very 

 small quantity of animal matter as a basis, and which 

 extends in every direction (i.e., in thickness, as well as in 

 length and breadth), its areolse or interspaces freely com- 

 municating with each other. These ' areolse,' and the 

 solid structure which surrounds them, may bear an ex- 

 tremely variable proportion 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 remark- 

 able lightness and porosity, if the network be a very open 

 one, 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 

 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 transparency is such, 

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

 tain two or three such layers, it is easy, by properly 

 4 focussing ' the microscope, to bring any one of them 

 into distinct view. From this very simple but very beau- 

 tiful arrangement, it comes to pass that the plates of 

 which the entire ' test ' is made up, possess a very con- 

 siderable 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." * 



To return, however, to our spine. When we look at it 

 * " The Microscope," 553. 



