INTEGUMENT 



359 



or other substances. Naturally, this work cannot be directly shared in 

 by any other tissue in the body. The surface cells must perform it, 

 assisted indirectly, in many cases, by the cells that lie next to and inside 

 of them. The simplest way in which this work is done is by some kind 

 of stiffening and hardening of the cells of the epithelium. We shall 

 study this method both in columnar and stratified epithelia. 



In the simplest columnar epithelia, this mechanical function is made 

 evident by the lengthening of the cell^ the formation of stiff fibrils in its 

 cytoplasm (to offset pressure), the thickening of its outer border (to resist 

 abrasion), together with the formation of terminal bars (to prevent the 

 entrance of harmful or needless fluids). 



These structures may be found performing their duties alone, as 

 in the covering epithelium of some flat worms (Planocera), or in com- 

 bination with many of the accessory structures 

 to be described hereafter. They can be studied 

 in a large number of the lower animals, and 

 we shall first examine them as seen in the integ- 

 ument of a turbellarian worm, Planocera fo- 

 lium (Fig. 325). 



Here two of the features mentioned above are 

 most excellently shown. The supporting func- 

 tion is performed by a series of stiff fibrils that 

 extend from the proximal surface of the cell to 

 its distal surface. Several granules may be seen 

 on each of the fibrils near its outer end, and at 

 the point where it leaves the surface a larger 

 granule is placed. The fibril is apparently con- 

 tinued directly through this granule to form one 

 of the cilia that, together with the other cilia 

 belonging to the same cell and to the other cells, 

 cover most of the body. 



Thus the supporting fiber is evidently used for at least one other pur- 

 pose than the more passive support of the cell. It is also used to support 

 the moving cilium, and it is possible that it also has structural features 

 that play a part in moving the cilium. It may even be conceived that 

 other cell-organs have important structural relations with the fibril. Such 

 ideas will not detract, however, from the conception of it as a cell-organ 

 of mechanical support. 



The fibril, so plainly seen as a straight support in this cell, is also found 

 in many other epithelial cells throughout the different epithelia. These 

 fibrils may be very slight, branched, net-forming, and otherwise variable, 

 and in some cases hard to distinguish from fibrils of a totally different 

 nature. 



FIG. 325. Protective and 

 ciliated epithelial cell from 

 the body surface of a flat 

 worm, Planocera. (After 

 SCHNEIDER.) 



