INTERNAL STRUCTURE : THE PROVENTRICULUS 469 



of the lower part of the cages, and could therefore pass from rat to 

 rat. Both cages are protected against the entrance of other fleas by 

 the muslin which envelopes the whole. 



THE INTERNAL ANATOMY. (PLATE LVIII) 



The internal structures of the flea are comparatively simple, and 

 resemble those of the Diptera. 



The alimentary canal (Plate LVIII, fig. 3) may be divided into pharynx, 

 oesophagus, proventriculus or gizzard, mid-gut or stomach, hind-gut, 

 and rectum. With the exception of the proventriculus 

 these parts resemble the corresponding ones in the alimentary 



Nematocerous Diptera. 



The pharynx is a chitinous chamber situated dorsal to the antennae 

 in the head. It is somewhat spindle-shaped and elongate, but is 

 expanded more on the dorsal than on the ventral side. It consists of two 

 chitinous plates, a dorsal and a ventral. The dorsal plate is continuous 

 with the epipharynx and the ventral with the hypopharynx, as in the 

 Diptera, but in the case of the latter organ the chitinous portions of the 

 two laminae are separated by a layer of fibrous tissue, in which the 

 salivary duct lies. Posteriorly the pharynx tapers off at the hind portion 

 of the head to become continuous with the oesophagus. The dilator 

 muscles pass between the dorsal plate and the wall of the head cavity, 

 and are separated into several discrete bundles. The oesophagus is 

 extremely thin, and has a wall consisting almost entirely of thin and 

 flexible chitin, with few cells and no muscular tissue. It extends as far 

 as the posterior limit of the thorax. 



The proventriculus is a remarkable organ, the function of which is 

 obscure. It is a short conical or button-shaped body, situated between 

 the oesophagus, the posterior end of which is dilated at the point of 

 junction, and the mid-gut, and guards the entrance to the latter. From 

 its inner surface there arises a close-set series of long chitinous rods (figs. 

 4 and 8), which pass first inwards and slightly backwards towards the 

 middle of the lumen, those from all parts of the periphery converging 

 towards one another, and then turn sharply backwards towards the lumen 

 of the mid-gut, into the anterior portion of which they project. The rods 

 are rounded on section, but are closely compressed together at the base, so 

 that they become hexagonal and produce a pattern on the wall like that 

 of the facets of the compound eye of Diptera. In longitudinal sections 

 of the organ they form a lattice pattern, due to the crossing of parts of 

 rods in front of and behind the bend. 



