EPITHELIA. 31 
2, FLAGELLATE OR CiLiate.—These cells are found in 
the protozoan classes Cv/iata and Mastigophora. The con- 
tractility is concentrated in the cilia or flagella, and the rest 
of the cell-body is often enveloped in a cell-membrane. 
In Metazoa free flagellate cells occur in the case of the 
male sexual elements or spermatozoa. Collared flagellate 
cells occur in great numbers in Forifera, whilst ciliated cells 
are commonly found in higher AZe/azoa, though not in the 
free condition. (See below.) 
3. QUIESCENT.—These are cells with no automatic 
movement ; they are usually enveloped in a cell-membrane 
which may assume the character of a cyst. They are 
usually spherical, or nearly so. Encysted Protozoa always 
Fig. 6.—QUIESCENT CELLS. 
I 2 
@ 
° ae 
1. Encysted Amceba. 2, Human red 3. Ovum. 
blood Corpuscles. 
assume this character, and some Jow organisms are per- 
manently in this phase. In JMetazoa free quiescent cells 
occur in the case of the eggs or female sexual elements, 
and in the “red corpuscles” of the blood. The former are 
usually spherical or oval, the latter flattened. 
Dependent cells of the A/efazoa are aggregated into 
masses or surfaces which are termed ¢éssues. A tissue is 
therefore an aggregate of cells which are alike in structure 
and function, 
We may recognise two sorts of tissue—(1) Tissues of 
two dimensions or surface-tissues (Epithelia) ; (2) Tissues 
of three dimensions or mass-tissues. 
"1. EprrHEtia.—An epithelium is, in its simplest condi- 
tion, of only one cell thick, but it has often several layers 
superposed :— 
(1) Ciliated epithelium is a common type, in which 
each cell has its outer end or surface covered with 
vibratile cilia. It is commonly found on the tentacles and 
