CLASSIFICATION OF ANIMAL PREPARATIONS. 



413 



m the manner recommended by Mr. Topping, in page 288 ; 

 the cover also should be very thin, as high powers will often 

 be required for their examination. Many species of the 

 following genera may be preserved in fluid, and some of them 

 even in Canada balsam : — 



Sponges. — These lowly organized animals 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 

 previous preparation, whilst others require either to be burnt 

 or acted on by acid, in order to display the small masses of 

 flint termed spicula, which form their rudimentary skeleton, 

 as well as other masses of the same material, which enter largely 

 into the frame-work of the young sponges or gemmules. The 

 British fresh water sponges abound frequently in gemmules, 

 but the spicula are mostly needle-shaped, like the raphides 

 in the hyacinth and squill, but they are not crystalline ; in 

 some of the marine species, especially those from Australia, 

 New Zealand, and Algoa-bay, most remarkable specimens of 

 both may be obtained. Mr. Bowerbank, who has paid 

 considerable attention to their microscopic structure, has 

 discovered a variety of new and interesting forms ; but as an 

 immense number of foreign species, which possess beautiful 

 spicula and gemmules, are still undescribed, it would be 

 impossible, at present, to give the names of more than a few 

 of the weU-known genera : — 



