50 



Mode of Formation of the Exoskelet.on. 



Originally the exoskeleton was believed to be pro- 

 duced by a secretion from the cells of the epidermis. 

 Yitzou, however, claimed that the process is effected in a 

 different manner. According to him the new shell is 

 produced in the following way. The contents of each 

 epidermal cell becomes modified at the outer margin. 

 This outer part becomes cut oft' from the rest of the cell. 

 Thus at this stage the epidermis is covered by a thin layer, 

 which, however is not one homogeneous whole, but is 

 divided up into numerous polygonal areas, each area 

 corresponding in shape and position to an epidermal cell. 

 The process is repeated ; the outer part of each cell is 

 again cut off, and at this stage we have a two-layered 

 polygonal cylinder above each cell. This process is 

 repeated until we have built up over each cell a multi- 

 layered cylinder. Since the cells of the epidermis lie 

 close together, the chitinous cylinders are also tightly 

 packed and form what appears to be a continuous 

 exoskeleton. The striations parallel to the surface of the 

 integument represent the successive lines of growth. 

 Thus, according to Vitxou, the process of formation of the 

 chitinous integument consists in the successive thickening 

 of the outer walls of the epidermal cells. 



Setae. 



These are long hair-like processes which project from 

 the exoskeleton in various regions of the body. In 

 sections each seta is seen to arise from the region of the 

 epidermis as a narrow tube enclosing a cavity. This tube 

 passes 'through the chitinous layers and projects from the 

 exterior as a long narrow process. Its walls are cuticular 

 and are continuous with the thin structureless cuticle 



