9O GENERAL PRINCIPLES OF ZOOLOGY. 



projections (cilia, or flagella) on the peripheral end of the 

 cells. Both are fine threads which arise from the body 

 of the cell, extend above the surface and here maintain 

 an extremely lively motion. In case of flagellated epithe- 

 lium (Fig. 24, d] each cell has only one vibratile projection, 

 but this is especially strongly developed ; in the case of 

 ciliated epithelium (Fig. 24, r), on the other hand, the sur- 

 face of the cell is covered with a thick forest of minute 

 threads moving in unison. 



Cuticle. The majority of the one-layered epithelia 

 are covered by a cuticle, a membrane which is secreted by 

 the epithelium cells in general, and hence very frequently 

 shows as polygonal markings the impression of the cells. 

 In many cases thin and inconspicuous, it may in other 

 instances become thickened into a very considerable layer, 

 much thicker than the matrix layer of epithelium which 

 secretes this cuticle. The cuticle then is evidently com- 

 posed of layers parallel with the surface, and forms a more 

 effective protection for the surface of the body than does 

 the epithelium ; it becomes a protective armor, as sho\vn, 

 among other examples, by the calcareous shells of mollusks 

 and the chitinous integument of insects (Fig. 24, _/"). 



Many-layered Epithelia. The protection furnished 

 by the cuticle in the case of the one-layered epithelium, 

 may in the many-layered be obtained immediately through 

 a chemical change of a part of the cells themselves. 



In the case of the many-layered epithelia the cells of 

 the various layers always can be distinguished by their 

 form : the deepest layer of cells consists of cylindrical cells ; 

 the superficial, on the other hand, of more or less flattened 

 elements; between lie several layers of transitional forms, 

 so that starting from the cylindrical cells we gradually pass 

 through the cubical cells to the flat cells of the surface. 

 As this arrangement shows, there exists a genetic connec- 

 tion between the cell-layers: the lower cylindrical cells are 

 in a state of active multiplication ; their descendants with 

 gradual changes of form push into the superficial layers, 



