64 The Larva of Chirononiiis 



miiscle-c-ells, each surrounding the oesophagus, and show- 

 ing a line of junction on the ventral side, where the ends 

 of the cell meet ; each cell contains a large nucleus. At 

 a later stage, the nucleus breaks up, and the whole cell- 

 substance divides into a number of striated fibres, Ijang 

 more or less parallel to the original cell. The wall of 

 each muscle-cell appears in transverse section as a clear, 

 wavy, fairly distinct line on the superficial side, but 

 somewhat difficult to determine on the deep side. The 

 longitudinal muscles of this part of the alimentary canal 

 are restricted to the neighbourhood of the upper bend ; 

 they are few, and lie outside the circular muscles, 

 stretching across the mouth of the blood-space from the 

 oesophagus to the cardiac wall (figs. 45, 46). 



' The oesophageal valve of Chironomus has the same 

 general arrangement as in Simuliam and many other 

 insects, but is complicated by secondary folds of the 

 epithelium and intima, which are the upper and loicer 

 in te rmedia fe hen ds. 



' In the reflected wall of the oesophageal valve the 

 behaviour of the different layers is difficult to determine; 

 but it is probably as follows : — ■ 



' I. The chitlnous intima continues to the upper bond, 

 where it ends in an uneven edge. At a distance from 

 the lower bend equal to about one-fifth of the total length 

 of the valve it becomes abruptly folded inwards and 

 backwards to the lower bend, thus forming the upper 

 intermediate bend. Arrived at this point, it is again 

 sharply reflected upwards (lower intermediate bend), and 

 lying jiarallel to its former course, passes straight to the 

 upper bend. Two of the three layers formed by this 

 repeated folding are closely applied to each other, but 

 between these and the third is a sj^ace, filled with a clear 

 coagulable fluid, which is not obliterated even when the 

 oesophagus is distended with food. This folding of the 



