320 EVENINGS AT THE MICEOSCOPE. 



find it honey-combed, and eroded, as it were, in this 

 remarkable fashion. 



Dr. Carpenter has described this texture so well, 

 that I shall not apologise for quoting his words to you, 

 especially as you will have an opportunity here of 

 testing their correctness, by personal observation. "It 

 is," he remarks, "in the structure of that calcareous 

 skeleton, which probably exists, under some form or 

 other, in every member of this class, that the micro- 

 scopist finds most to interest him. This attains its 

 highest development in the JSchinida, in which it 

 forms a box-like shell, or ' test,' composed of numerous 

 polygonal plates jointed to each other with great ex- 

 actness,. 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 evei-ywhere the same : for it 

 is composed of a network, 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 

 areolae or interspaces freely communicating with each 

 other. These 'areolae,' and the solid structure , which 

 surrounds them, may bear an extremely variable pro- 

 portion one to the other ; so that, in two masses of 

 equal size, the one or the other may greatly predom- 

 inate ; and the texture may have either a remarkable 

 lightness and porosity, if the network be a very open 

 one, or may possess a considerable degree of compact- 

 ness if the solid portion be strengthened. Generally 

 speaking, the different layers of this netwoi'k, which 

 are connected together by pillars that pass from one 



