55 



(fig-. 39). They consist essentially of two transverse 

 ridges of tissue, very similar, histologically, to the palps, 

 but without the grooves. 



These two ridges, however, are produced in certain 

 places into prolongations, which, as well as the free edge 

 of the lips generally, divide and re-divide into very small, 

 short and swollen processes. In this way the lips come 

 to have a dendritic appearance. There are two main 

 prolongations of the upper lip with dendritic margins, 

 situated one at each side near the origins of the labial 

 palps. The lower lip has a very large median prolonga- 

 tion which interlocks with those above, and lesser ones 

 along the sides. It seems, therefore, that this develop- 

 ment of the branched margins is for the purpose of closing 

 over the mouth, leaving a channel which communicates 

 at each side with the groove between the two labial palps. 



The margins of the lips are deeply pigmented with 

 the same orange-yellow that is found on the palps, and 

 also lining the oesophagus. 



The upper surface of the upper lip and the lower 

 surface of the lower lip are bounded by a layer of 

 epithelial cells, which are almost cubical in shape, and 

 are crowded with pigment granules, especially near the 

 surface. This layer is continuous with the somewhat 

 similar layer that covers the outer surfaces of the labial 

 palps. The surfaces of the upper and lower lips, which 

 face one another and are continuous with the grooved 

 surfaces of the labial palps, are bounded by a layer of 

 much elongated columnar cells, which contain no 

 pigment, but bear numerous cilia. The structure between 

 these two layers is loosely packed connective tissue, with 

 numerous spaces c(uitaining blood corpuscles. A slightly 

 denser layer immediately underlies the epidermis. 



The mouth itself (fig. 39, M.) is a transverse slit 



