232 
Argas persicus 
what Christophers^ has termed the jyroventriculm' fold, and this 
structure evidently fulfils a similar function to the corresponding 
structure which is found in many suctorial Hexapoda. 
The Oesophagus. 
Part I, Text-fig. 2 ; Part II, Plates XIV, XV and XVI, oes. 
The oesophagus is a soft-walled tube of small calibre, which connects 
the pharynx with the stomach. Its course is not tpiite direct, for, 
immediately at its commencement at the posterior end of the phar 3 mx, 
it bends abruptly downwards and slightly forwards; then almost im¬ 
mediately it again turns backwards, forming a short S-shaped bend before 
continuing its course through the brain, entering this organ near the 
anterior end of its ventral surface, and emerging on the middle of the 
dorsal surface to enter the stomach almost directly. At its proximal 
extremity, the oesophagus protrudes for a short distance into the lumen 
of the stomach, where it forms the previously mentioned 'proventricidar 
fold. 
In cross-section, the oesophagus is circular. Its walls consist of an 
epithelial la^^er of irregularly columnar cells with small nuclei, sur¬ 
rounded by a thin muscular layer composed for the most part of 
circularly arranged fibres, a few longitudinal strands of muscle fibre 
appearing in the peripheral portions. The lumen of the oesojrhagus is 
lined with a delicate chitinous intima, which, together with the under¬ 
lying epithelium is thrown into shallow longitudinal folds. The 
proveutricular fold is composed of the same elements as the rest of the 
oesophagus, but the epithelial layer, which becomes enormously thickened, 
is here composed of elongate columnar cells, which merge abruptly into 
the epithelium of the stomach wall. The chitinous lining of the 
oesophagus is continued over the internal surface of the proventricular 
fold but terminates at the point where its epithelium joins that of the 
stomach. 
The Stomach. 
Plates XIV and XV ; Plate XVI, figs. 1-6 : st, cc. al. 
The stomach with its coecal appendages forms the greater part of 
the alimentary canal and is, indeed, the bulkiest organ in the body of 
the tick. It consists of a median portion—the stomach proper (see 
' Christophers, S. E. (1906), p. 31. 
