23 



reagents ; it appears as a syncytium containing' oval nuclei 

 dispersed at frequent intervals, and is similar in its 

 structure to the liypodermis, with which it is directly con- 

 tinuous (figs. 27 and 28). As Fernald remarks, there are 

 slight depressions in the chitinous lining which alternate 

 in position with the nuclei and may perhaps indicate the 

 boundaries between adjacent cells. Externally, the 

 epithelial layer rests on a delicate basement membrane 

 [b. m. in fig. 29), and immediately outside the latter is a 

 strongly developed layer of circular muscle fibres (figs. 27, 

 28 and 29). The structure of these fibres is remarkable; 

 they are all cross striated, and each fibre is in the form of 

 a ring completely encircling the gut, with its two ends 

 meeting in the mid-dorsal line. The sarcoplasm of each 

 fibre lies wholly external to' the contractile substance, 

 and in it is situated the nucleus of the fibre (fig. 29). The 

 nuclei of the fibres are all situated in the dorsal line, and 

 they form a continuous row, stretching right along the 

 fore-gut. Each muscle fibre is inverted externally by an 

 extremely delicate sarcolemma {sbnma in fig. 29). 



In relation with the j)harynx and a small tract of the 

 adjacent portion of the oesophagus a remarkable system 

 of radial muscles is developed and, furthermore, the 

 circular fibres are more strongly developed in this region 

 than elsewhere. These muscles fall into two groups, 

 viz., a dorsal or anterior series and a ventral or posterior 

 series (Plate III., fig. 27). The muscles forming these 

 two groups take their origin in the cuticle of the body 

 wall. Those of the dorsal series cross the cavity of the 

 head in an oblique direction and, passing between the 

 circular fibres, attach themselves to the chitinous intima 

 of the gut. In the case of the ventral series, they cross the 

 head cavity in the horizontal plane, and converge to 

 become inserted into the tentorium. A similar system of 



