18 W. BALDWIN SPENCER. 



backwards. This tooth-like body (To.) is a very distinct struc- 

 ture when the mouth and phaiynx are examined by means of 

 sections, or when they are removed from the body and examined 

 whole. In some cases even it separates off from the rest of the 

 cuticle. It is, however, a single median and not a paired 

 structure (unless, which is very improbable, it be formed of 

 two fused lateral halves), and though it certainly suggests the 

 presence of a modified mouth appendage, its very distinctly 

 unpaired nature must, in the absence of any evidence as to its 

 double nature, be taken as negativing the idea of its representing 

 such a structure. 



At its upper end a curious modification takes place in the 

 wall of the pharynx. The epithelial layer on the inner side, 

 which is elsewhere thin, becomes here much thickened and 

 swollen out (fig. 32, P. E.^). In addition to this there is 

 present (fig. 32, P. E.^), just where the pharynx turns back- 

 wards, a special layer which lies external to the ordinary epi- 

 thelial cells and stains deeply. It is apparently cellular in 

 nature, and secretes on its aspect facing into the pharynx a 

 very thin chitinous coat. The outer half of the upper end of 

 the tooth-like structure ends abruptly against this layer, but 

 the inner half of the same structure is continuous with a thin 

 process of chitin which runs up between the external deeply 

 staining layer and the ordinary epithelium. The exact relation- 

 ship of the parts is shown in the figure, and it would seem as if 

 this special structure were intimately associated with the growth 

 of the tooth-like organ in the hinder wall of the pharynx. 



The anterior wall of the latter has, for the most part, only 

 a comparatively thin layer of cuticle, beneath which is the 

 layer of epidermic cells [E.) amongst which the muscle-fibres 

 are inserted. The cells are packed somewhat closely together, 

 and appear to lose their distinctly columnar nature. Imme- 

 diately opposite to the pad-like structure in the posterior wall 

 the cuticle becomes somewhat thickened. On the posterior 

 wall, on the contrary, the cells are very distinctly columnar ; 

 in fact, just where the cuticle bends inwards from the external 

 surface at the mouth the cells, which over the body surface arc 



