STRUCTURE OF PROTOZOA. 
135 
polypes; and new branches, are formed either by the sub¬ 
division of the polypes, or by gemmation from the connecting 
substance. 
128. When we pass from Zoophytes to animals of still 
simpler organization, we lose all trace of definite symmetry, and 
find ourselves amid forms which cannot be referred to any 
particular plan of growth. These, moreover, are for the most 
part distinguished by an extreme simplicity of structure ; no 
such differentiation of parts exhibiting itself among them, as 
is shown in the “ organs ” of even the simplest Zoophyte or 
Worm. Hence they are appropriately designated Protozoa. 
They may, in fact, be considered as essentially consisting of 
homogeneous particles of a jelly-like substance, to which the 
name of Sarcode has been given; and the chief modification 
this undergoes, consists in the consolidation of certain parts 
of it by the deposit of horny, calcareous, or siliceous matter, 
so as to form a skeleton. This may take place on the outer 
surface only, so as to form shells very like those of Mollusks 
in miniature, as we see among Foraminifera (fig. 78); or it 
may occur in the midst of the fleshy substance, so as to 
form an internal network, such as presents itself in the 
Sponge. The endowments of the “ sarcode ” are very extra¬ 
ordinary ; and will be best understood by observation of the 
life-history of one of those simplest Protozoa, in which the 
whole body consists of but a minute particle of it. 
Fig. 77.— Riiizopoda :—A, Amceba ; B, Actinophrys. 
129. Such an example is afforded by the Amoeba (fig. 77 a), 
—a creature frequently to be met with in great abundance 
in fresh and stagnant waters, vegetable infusions, &c. Its 
