AMGBA. 89 
have a definite rind, which in a large number (Infusorians) 
bears motile cilia or flagella, but in the others (Sporozoa) 
is usually without locomotor structures. But these three 
phases—ameboid, ciliate or flagellate, and encysted—may 
occur in the life history of one form; and the three main 
lines of evolution—Rhizopods, Infusorians, and Sporozoa—are 
marked by the predominant occurrence of the ameboid, ciliate 
or flagellate, and encysted phase of cell life. Many have a 
skeletal framework,—of lime, flint, or other material,—while 
within the cell there ts a special kernel or nucleus, or there 
may be several. There are also other less constant structures. 
A Protozoon multiplies by dividing into two daughter units, 
or into a large number; and two individuals often unite, 
temporarily or permanently, in conjugation, which is analogous 
to the union of ovum and spermatozoon in higher animals. 
A few types, instead of remaining single cells, form by 
dtuiston or budding loose colontes, taking a step, as it were, 
towards the Metazoa, but never forming differentiated tissues. 
first Type of Protozoa—AM@BA 
Ameba, a type of Rhizopods, especially of those in which 
the outflowing processes of living matter (pseudopodia) are 
blunt and finger-like (Lobosa). 
Description.—Ameba proteus and some other species are 
found in the mud of ponds; 4. ¢erricola occurs in damp 
earth. Some are just large enough to be seen with the 
unaided eye. The diameter is often about one-hundredth 
of aninch. Each is a unified corpuscle of living matter, 
and glides over the surface of stone and plant by protruding 
and retracting the pseudopodia. As they move the shape 
constantly changes, whence the old (1755) name of “ Proteus 
animalcule.” Round the margin, which may show an 
apparent radial striation, the cell substance is firmer and 
clearer than it is in the interior, where it is more fluid, but 
contains very abundant granules, some of which are of 
a proteid, and others of a fatty nature. In the centre of 
the cell lies the usually single nucleus. The food consists 
of minute Alga, such as diatcms, or of vegetable débris. 
There is reason also to suspect cannibalism. The food 
is surrounded by the finger-like processes, and engulfed 
