90 THE OCEAN WORLD. 



out for the purpose in the salt marshes near the sea-shore in the 

 south of France and elsewhere — the salt water often reaches a very- 

 high degree of concentration, and is then found to acquire a fine rose 

 colour, which is due to the presence of innumerable masses of small 

 Infusoria or Crustacea having a reddish hue. At one time it was 

 thought that the Tripoli stone was formed of tens of millions of 

 the cases of Infusoria ; but it is now known that this stone contains 

 vast masses of sponge spicules and Diatomacese. 



The study of these creatures is intensely interesting to the 

 naturalist, the philosopher, the physician, and the general reader. 

 They have had a great part assigned to them in Nature, as is evident 

 in the formation of certain beds of rock of immense extent, in which 

 the geologist traces their action. 



Our earliest knowledge of the Infusoria is traceable to the seven- 

 teenth century. In 1752 Hill essayed the first attempt at their 

 classification. In 1776 O. F. Miiller gave them the name of Infu- 

 soria, because he found them in such great abundance in animal and 

 vegetable infusions. To the celebrated naturalist Leuwenhoek we 

 are indebted for much information about tliem. Miiller published 

 a special book upon them. 



From that time the Infusoria have been considered as forming a 

 special group among the Protozoa ; afterwards, in the pages of von 

 Baer and of De Blainville, we see that they regarded these creatures, 

 so imperfect in appearance, as only the indeterminate prototype of 

 other classes. But ideas changed altogether respecting them when 

 microscopes well supplied with achromatic lenses were employed in 

 their study. Thanks to the labours of Ehrenberg and Dujardin, 

 Stein and Claparade, we have now arrived at a better comprehension 

 of the organisation of these infinitely small beings ; and naturalists 

 have established, with more exactness, the limits of the zoological 

 group to which they belong. 



Some waters are so filled with Infusoria that it is only necessary 

 to dip at random into the licpiid medium to procure them in abund- 

 ance. In other waters they form a stratum, occupying the whole 

 basin. In general, however, it is necessary to search for them where 

 the water is calm, and filled with vegetation of some kind, such as 

 Conferva, or Lcmtia, &c., in the marshes, and Algce if in the sea. 

 Certain Infusoria live not only in water, but also in places habitually 

 moist, as among tufts of mosses, on moist soil, or on damp walls. 

 Others live as parasites on the exterior or sometimes in the interior 

 of animals. 



But, as their name indicates, they will be found in all aqueous 



