EPI7HELIA. 31 



2. Flagellate or Ciliate. — These cells are found in 

 the protozoan classes Ciliata 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 Porifera^ whilst ciliated cells 

 are commonly found in higher Metazoa^ 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. 

 \ 23 



I. Encysted Amoeba. 2. Human red 3. Ovum, 



blood Corpuscles. 



assume this character, and some low organisms are per- 

 manently in this phase. In Metazoa 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 Metazoa are aggregated into 

 masses or surfaces which are termed tissues. A tissue is 

 therefore an aggregate of cells which are alike in structure 

 and function. 



We may recognise two sorts of tissue — (i) Tissues of 

 two dimensions or surface-tissues (Epithelia) ; (2) Tissues 

 of three dimensions or mass-tissues. 



I. Epithelia. — An epithelium is, in its simplest condi- 

 tion, of only one cell thick, but it has often several layers 

 superposed : — 



(i) 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 



