EPITHELIA. 277 



describes these as five extraordinarily large cells, four of 

 which lie close together, commonly forming a rectangular cord, 

 whilst the fifth is separated from the rest by a cell's diameter, 

 and always contains two nuclei; the others have only one in 

 each cell. 



This description may be applied to the same organs in the 

 larva of Musca, except that all the cells usually have twa 

 nuclei or more, and I am by no means clear that the number 

 is constant. They undergo complete histolysis in the young 

 nymph. 



I am greatly tempted to regard these remarkable structures 

 as the remains of segmental organs, similar to those of the 

 Chaetopods and Peripatus. Although their developmental 

 history is unknown, Graber gives a figure [114, Fig. 128] of ' the 

 lateral body cavity ' of Stenobothrus variabilis, in which the 

 cavity, lb, W, is apparently a segmental organ, and the cells, o', 

 are, I suspect, Wiclowiejski's oinocytes and represent the 

 remains of a segmental tube. 



3. EPITHELIA. 



Epithelial Cells differ from those of the parablast in being 

 developed directly from the outer or inner layers of the blasto- 

 derm, and in not exhibiting an amoeboid stage. The individual 

 cells are generally united by a scanty intercellular substance, 

 termed cement substance. It is usual to distinguish the several 

 varieties by their form, as tabular, or flattened, spheroidal and 

 columnar cells. An epithelium may consist of a single layer of 

 cells, when it is termed simple, or of several layers, when it is 

 said to be stratified. 



Many epithelial ceils possess cilia, but such are never found 

 in the Arthropoda. 



The epithelial cells of Insects may be classed as chitino- 

 genic, secreting, and absorbent cells, on the one hand, and 

 sensory cells on the other. 



The cells derived from the epiblast are chiefly chitinogenic ;. 

 those from the hypoblast arc secreting or absorbent. They 



