14 PERIPATUS 



but is everywhere, with the exception of the perioral region, 

 raised into minute secondary papillae, which in most instances 

 bear at tlieir free extremity a somewhat prominent spine. The 

 whole surface of each of the secondary papillae just described is 

 in its turn covered l;iy numerous minute spinous tubercles. 



The epidermis, placed immediately within the cuticle, is 

 composed of a single layer of cells, which vary, however, a good 

 deal in size in different regions of the body. The cells excrete 

 the cuticle, and they stand in a very remarkable relation to the 

 secondary papillae of the cuticle just described. Each epidermis 

 cell is in fact placed within one of these secondary papillae, so 

 that the cuticle of each secondary papilla is the product of a 

 single epidermis cell. The pigment which gives the characteristic 

 colour to the skin is deposited in the protoplasm of the outer ends 

 of the cells in the form of small granules. 



At the apex of most, if not all, the primary wart-like papillae 

 there are present oval aggregations, or masses of epidermis cells, 

 each such mass being enclosed in a thickish capsule and bearing a 

 long projecting spine. These structures are probably tactile organs. 

 In certain regions of the body they are extremely numerous ; more 

 especially is this the case in the antennae, lips, and oral papillae. 

 On the ventral surface of the peripheral rings of the thicker 

 sections of the feet they are also very thickly set and fused together 

 so as to form a kind of pad (Figs. 6 and 7). In the antennae 

 they are thickly set side by side on the rings of skin which give 

 such an Arthropodan appearance to these organs in Periimtus. 



The Tracheal System. 



The apertures of the tracheal system are placed in the depres- 

 sions between the papillae or ridges of the skin. Each of them 

 leads into a tube, which may be called the tracheal pit (Fig. 10), 

 the walls of which are formed of epithelial cells bounded towards 

 the lumen of the pit liy a very delicate cuticular membrane con- 

 tinuous with the cuticle covering the surface of the body. The 

 pits vary somewhat in depth ; the pit figured was about O'OQ mm. 

 It perforates the dermis and terminates in the subjacent muscular 

 layer. 



Internally it expands in the transverse plane and from the 

 expanded portion the tracheal tuljes arise in diverging bundles. 

 iN^uclei similar in character to those in the walls of the tracheal 



