662 MEDICAL ENTOMOLOGY 



end, which is U-shaped in cross-section, it opens into the buccal cavity 

 at a right angle to the ventral wall of the latter (Plate LXXXV, fig. 1) ; 

 The pharynx the P enin S consists of a V-shaped slit, the lateral 

 arms of which are directed backwards. After the initial 

 bend the tube passes first upwards and then straight backwards to 

 become continuous with the oesophagus. In the main part of the tube 

 the lumen is X-shaped on section (Plate LXXXV, fig. 5), with two dorsal 

 and two ventral horns. The musculature is complex, and consists of 

 sets of dilator muscles, passing between the dorsal and ventral walls of 

 the capitulum and the plates which make up the sides of the pharynx, 

 and in addition certain accessory bundles, which appear to act as 

 sphincters. These latter pass between the dorsal and ventral horns on 

 each side, and between the dorsal and ventral horns of the two sides in a 

 transverse direction. In Ixodes r lei mis, according to Samson, the struc- 

 ture of the pharynx is further complicated by the bifurcation of the 

 dorsal horns, the secondary horns being also connected by small muscle 

 bundles. The mechanism of the pharynx appears to be of the usual 

 type, the dilator muscles, by withdrawing the plates from one another, 

 producing the requisite negative pressure, while the sphincter muscles, 

 acting in a peristaltic manner, assist in the propulsion of the fluid into 

 the succeeding part of the gut. 



At the level of the posterior end of the mandibles, which, as noted 

 above, project into the body cavity, the pharynx merges into the 



oesophagus ; this at once dips downwards almost at a 

 The oesophagus. . , , . 



Plate LXXXIII, fig. 3 n g nt angle and passes through the brain, emerging at 



its posterior border. After a short upward course it 

 enters the mid-intestine on its ventral surface. 



The oesophagus is lined by single layer of high columnar epithelial 

 cells, which are covered internally by a chitinous intima. External to 



the delicate basement membrane there is a layer of 

 Histology of the , , , ... . . 1,1 



intestinal canal Clrcu lar muscle fibres. At the point where the oeso- 



and its appendages phagus enters the mid- intestine the cells are heaped up 

 to form a rudimentary proventriculus, and the circular 

 muscle fibres are more abundant ; the cells of the oesophagus end abrupt- 

 ly at its junction with the mid-intestine. The mid-intestine and all the 

 diverticula are lined by a single layer of somewhat irregularly shaped 

 cells (Plate LXXXVI, fig. 1), the shape of which depends to a large extent 

 on the stage of digestion. They may be either flattened out to form a 

 continuous layer as in full-fed specimens, or they ma)- exhibit all sizes 

 and shapes and project from the wall, as in specimens in which digestion 



