INTEGUMENT 



361 



its place is taken by a new cuticle that is usually formed before the old 

 one is removed. 



The substance of the cuticle is an organic material called keratin, 

 which varies somewhat in the different forms. It is, in some cases, said 

 to be made of cellulose. The cuticle found on many worms is a simple 

 type, of moderate development, and that of the earthworm will do for 

 examination. 



The cuticle of the earthworm is a layer of material about one eighth 

 or one tenth of the thickness of the layer of epithelial cells. Its structure 

 does not appear to advantage in a section, and it is best studied in a piece 

 of the layer that has been torn or macerated off in alcohol of 30 per 

 cent (Fig. 327). Here it is seen that the whole structure of this organ 

 consists of two parallel series of 

 fibrils, each set lying at right an- 

 gles to the other, and all flat- 

 tened together by a cement sub- 

 stance that fills all the interstices. 

 At somewhat regular intervals 

 the fibrils are pressed somewhat 

 apart by a greater amount of the 

 cement substance, and through 



FIG. 327. Superficial view of cuticle from an 

 earthworm. X 1500. 



this thickened cement may be 

 seen a fine pore passing through 

 the entire layer. This pore is seen in transverse sections of the cuticle 

 in situ on the epithelium, to provide a passage for the secretion of one 

 of the many unicellular mucous gland-cells that are to be found at nu- 

 merous intervals on the surface of the body. 



Certain regions of the cuticle show spots that provide many similar 

 pores set closely together. Again, the section of the entire integument 

 will show that these are the passages through which the perceptory 

 endings of a number of nerve cells come to the surface where they will 

 be in contact with the outer conditions. 



The fibrils that make up the bulk of the cuticle are straight and par- 

 allel, and run, as has been said, in two series, the strands of each series 

 being at right angles to the strands of the other. Both series are sym- 

 metrically oblique to the main axis of the animal's body, and are thus at 

 an angle of forty-five degrees to it. The fibrils are thought to pass from 

 one surface to the other, and to interweave. It is puzzling to understand 

 how the epithelial cells that are arranged all over the surface of the body, 

 and in no particular order, are able to cooperate in such a manner as to 

 lay down or form such long, straight, and parallel fibers in two distinct 

 sets. 



The exact method by which the cuticle is laid down by the cells has 



