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 Diatomacez. 
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 zzfusions. To the celebrated naturalist Leuwenhoek we 
are indebted for much information about them. 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 liquid 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 
Conferve, or Lemna, &c., in the marshes, and Adee 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 
