Oesophagus 53 



against tlie toothed labium, or cramming it into tlie 

 mouth. In the floor of the mouth above the mentum 

 (see p. 29) is a small forward projection, the lingua, 

 behind which the salivary ducts open. 



The oesophagus or gullet is a straight and narrow tube Oesopha- 

 of simple structure. It is lined by an epithelium (not 

 easily seen, and often only to be discerned by the 

 cell-nuclei) and a chitinous cuticle, which is sharply 

 folded lengthwise, so that the enclosed cavity is almost 

 obliterated except when food is actually passing along 

 it '. Outside the epithelium comes a muscular coat, 

 invested inside and out by a thin membrane, which 

 sometimes becomes separated in a macerated gullet. The 

 muscular coat consists of a number of transverse rings, 

 each of which is a cell, with thin cell-wall and nucleus. 

 The ends of the cells are in contact on the ventral 

 side, and form oblique sutures. In optical section they 

 often look deceptively like an epithelium. Each cell, 

 e:5^cept in very young larvae, encloses a skein of 

 fibres, which show cross-striation. The fibres lie in 

 the direction of the length of the cell, i. e. at right 

 angles to the oesophagus'". In the head the gullet is 

 held in place by several pairs of slender muscles, which 

 pass downwards and forwards from the occipital region 

 (fig. 19). A small pouch extends forwards from its dorsal 

 wall near the mouth. 



The dilatations of the oesophagus, known as crop and Oesopha- 

 gizzard, which in many insects and myriopods facilitate 

 a process of oesophageal digestion, as explained by 

 Plateau -^ do not occur in the Chironomus-larva. The 



' In a cast skin the chitinous lining is drawn out, and remains attached 

 to the skeleton of the head as a long crumpled band. 



2 A much finer striation, which we believe to be due to local and tem- 

 poraiy aggregations of the cell-protoplasm, often forms across the cell 

 from side to side. This is shown in fig. 52. 



= 1875, 1878. 



