14 ANIMAL FORMS 
esting additional characters. For example, the form repre- 
sented in Fig. 5, C, constructs a sac-like skeleton of tiny 
pebbles cemented together, into which it may withdraw for 
protection. Others construct similar envelopes of lime or 
flint, and still others, as they continue to grow, build on 
additional chambers, giving rise to a great variety of forms 
often of wonderful beauty. In the tropics, particularly, 
some of the shelled Protozoa are so abundant that they may 
impart a whitish tinge to the water, and in some places 
their empty shells on falling to the bottom form immense 
deposits. The chalk cliffs of England are in large measure 
made up of such shells. 
16. The Infusoria.—A little over two hundred years ago 
it was discovered that wherever water remained stagnant it 
became favorable for the rapid multiplication of a large 
number of species of Protozoa which live in such situations. 
These are known as Infusoria, and, like the preceding spe- 
cies, are usually of microscopic size and of the most varied 
shapes. The first striking feature of their organization is 
the presence of a delicate though relatively firm external 
cell membrane known as the cutzele, which preserves a defi- 
nite shape to the body. Such a method of locomotion as 
exists in the preceding group is consequently an impossi- 
bility, but other and more highly developed structures per- 
form the office. These latter organs are of two types, and 
their general characteristics may be readily understood 
from an examination of a few species living in the same 
localities as the Amba. 
1%. The Euglena—The first type exists in the common 
fresh-water organism known as Euglena, represented in 
Fig. 6, A. Here the spindle-shaped body is surrounded by 
a delicate cuticle perforated at one point, where a funnel- 
shaped depression, the gullet, leads into the soft proto- 
plasmic interior. From the base of this depression the 
protoplasm is drawn out in the form of a delicate whip-like 
process known as the flagellum. This structure, always 
