ONE-CELLED ANIMALS AND SPONGES. 123 
nouth for taking food and the two kinds of cilia, one 
for locomotion and one for producing currents of water 
to drive food into the 
mouth. Even a single 
cell then may have its 
parts specialized for per- 
forming different kinds of 
work. 
Characteristics of the 
Protozoa. The Protozoa 
are minute animals 
having but a single cell 
of protoplasm, moving by 
pseudopodia, or cilia, and 
reproducing withouteggs. 
Sponges. Itis not rec- 
ommended that sponges 
be studied in the labora- 
tory in an elementar 
y y Fic. 107. ~A Paramecium (highly 
course, but for purposes magnified).  0.g., oral groove; 
of comparison it is neces- ph, pharynx. 
sary to become familiar 
with the most important facts concerning their structure. 
Sponges are composed of many cells but slightly 
specialized. A single sponge really seems almost as 
much like a colony of Protozoa as like a distinct animal. 
The outer layer of cells which is simply a sort of skin 
is called the ectoderm, the inner layer or lining of the 
cavities of the body is called the exdoderm. Between 
the ectoderm and the endoderm lies the mesoglwa, in 
which the skeleton is produced. The flesh of the 
sponge taken altogether is called sarcode. The com- 
mon bath-sponge as we use it is only the skeleton. 
We may imagine that the hard parts we see have once 
been imbedded in fleshy matter (mesoglcea); that the 
fleshy matter was covered with a skin (ectoderm); and 
that the cavities so apparent in the skeleton were lined 
with another skin (endoderm). 
