Natural History of the Animalcules. 115 
applied, owing to the fa<5t that the pseudopodia are in 
the form of radiating threads. They possess a shell or 
skeleton, but this is composed, not of lime as in the 
reticulate Animalcules, but of flinty or siliceous matter; 
while radiating siliceous spines are very charafteristic of 
the group. The various members of the group differ 
considerably from each other — an extreme point being 
reached in those organisms in which a simple form be- 
comes composite or multicellular. In these latter, how- 
ever, there is no differentiation in function, there being 
no difference in position among the cells : each one cell 
performs the same funftions as every other cell, unlike 
the case of the higher organisms where the various cells, 
differing in position, perform different functions accord- 
ing to their position. These radiate Animalcules are 
included under the group name of Radiolaria. They 
are generally microscopic, but the composite forms grow 
to more than half-an-inch in diameter. They are, with 
few exceptions, marine forms, abounding in the surface 
waters, though also common in the deep sea. A large, 
freshwater group, called Sun-animalcules from their 
typically radiate pseudopodia, are generally separated 
from the other Radiolaria. 
The next and last type that presents itself for our 
notice, is one to which the highest place as regards 
organisation must be given. This type belongs to a 
group to which reference has already been made, viz. the 
Infusoria, In these we lose sight altogether of the 
temporary protrusions of protoplasm, the pseudopodia, 
which are so charafteristic of the other Animalcules — 
I here the processes of the protoplasm are permanent. 
They take the form either of minute, rapidly vibratile 
P2 
