320 EVENINGS AT THE MICROSCOPE. 



find it honey-combecl, and eroded, as it were, in tliia 

 remarkable fashion. 



Dr. Carpenter has described this texture so -;\'ell; 

 tliat 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 flie micro- 

 scopist finds most to interest him. This attains its 

 highest development in the Eddnida^ 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 ever^nvhere tlie 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 (z. e. 

 in thickness, as well as in length and breadth), its 

 areolae or interspaces freely communicating Avith each 

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

 surrounds them, ma}' 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 eitlier 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 difi'erent layers of this network, which 

 are connected together by pillars that pass from one 



