86 THE MICROSCOPIST. 



feet thick, consists almost entirely of the siliceous shells of 

 Infusoria, so small that forty thousand millions are contained 

 in a single cubic inch. 



The eatable earth of Sweden and Lapland is likewise com- 

 posed mostly of such shells. A layer of this earth occurs in 

 the province of Luneberg, Saxony, which is twenty-eight feet 

 thick. It contains a beautiful species of minute, oval, figured 

 shell called the Campilodiscus. 



Sponges. These lowly-organized bodies are found both in 

 salt and fresh water in all parts of the globe. Many of them 

 are very minute, and may be examined without much prepa- 

 ration, but others require to be burned, or acted on by acid, 

 to show the small masses of flint, called spicula, which form 

 their rudimentary skeleton, as well as other masses of the 

 same material, which enter largely into the framework of the 

 young sponges or gemumles. 



Corals are best examined by horizontal and vertical sec- 

 tions. If the animal matter only is required, the sections may 

 be macerated in hydrochloric acid, to which five or six times 

 its bulk of water has been added. 



Zoophytes. Besidents at the sea-side, or occasional visitors, 

 when provided with a microscope, have frequent opportunities 

 of examining some of these most elegant of animal forms. 

 Scarcely a piece of sea-weed or a fragment of shell will be 

 found, that does not afford a habitation for some member 

 of this interesting family. The animals are generally found 

 in clusters or compound ; sometimes communicating at a com- 

 mon centre; at other times distinct and only connected by 

 the solid matter of which their polypidoms are formed. Some 

 few, as the common fresh-water polype, do not secrete any 

 hard substance either around or within them. 



INSECTS. These afford the most numerous and beautiful 



