60 THE HISTORY OF CREATION, 
Fic. 14.—Navicula hippocampus (greatly magnified). 
In the middle of the cell the cell-kernel (nucleus) is 
visible, together with its kernel speck (nucleolus). 
The Flint-cells (Diatomeze), a sixth class of 
Protista, are perhaps the most closely related 
to the Labyrinthuleze. These primary crea- 
tures—which at present are generally con- 
sidered as plants, although some celebrated 
naturalists still look upon them as animals— 
inhabit the sea and fresh waters in immense 
masses, and offer an endless variety of the 
most elegant forms. They are mostly small microscopic 
cells, which either live singly (Fig. 14), or united in great 
numbers, and occur either attached to objects, or glide and 
creep about in a peculiar manner. Their soft cell-substance, 
which is of a characteristic brownish yellow colour, is 
always enclosed by a solid and hard flinty shell, possessing 
the neatest and most varied forms. This flinty covering is 
open to the exterior only by one or two slits, through 
which the enclosed soft plasma-body communicates with 
the outer world. The flinty cases are found petrified in 
masses, and many rocks—for example, the Tripoli slate 
polish, the Swedish mountain meal, etc.—are in a great 
measure composed of them. 
A seventh class of Protista is formed by the remarkable 
Slime-moulds (Myxomycetes). They were formerly uni- 
versally considered as plants, as real Fungi, until ten years 
ago the botanist De Bary, by discovering their ontogeny, 
proved them to be quite distinct from Fungi, and rather 
to be akin to the lower animals. The mature body is a 
