The Hog Louse 677 



In cleared specimens he evidently saw onlj' the anterior, strongly chitinized 

 band of the pharynx. It ia a somewhat cone-shaped structure having its 

 widest diameter, wiiich is approximately 0.15 millimeter, a little posterior 

 to its transverse median line. In sections its ventral aspect is seen to 

 lie almost level along the median longitudinal line of the head, and its 

 dorsal surface passes obliquely toward the top of the head. Between its 

 transverse median line and its part of greatest diameter is a more strongly 

 chitinized region crossing the dorsal surface as a band and passing obliquely 

 and posteriorly down the sides to the ventral surface, where the two 

 bands run backward for a short distance, each lying somewhat laterad 

 of the median line (Plate LX,3). Behind the muscle insertions is a second 

 region of strong chitinization, followed by a sphincter muscle, behind 

 which the diameter lessens until it passes as the slender esophagus under 

 the brain. 



The esophagus 



The esophagus (Plate LX, 2 and 3) passes directly backward between 

 the tritocerebral lolx-s of the brain, over the sub-esophageal ganglion, 

 and into the thorax between the two main tracheal trunks. At the 

 posterior end of the head the esophagus, the dorsal vessel, the tracheae, 

 and the connectives between the sul)-esophageal and thoracic ganglia, 

 are inclosed by a wall of thin cuticula, which is continuous with and shows 

 the same staining reactions as the cuticula separating the posterior end 

 of the piercer sheath from the thorax. It is a structureless membrane 

 (Plate LX, 5). At its posterior end the esophagus passes over the anterior 

 part of the stomach lying in the thorax, and enters its dorsal surface 

 under the tergite of the second abdominal segment. Its length from the 

 posterior end of the true pharynx to its passage into the stomach is 

 approximately 1.03 millimeters and its diameter 0.03 millimeter. In 

 sections its wall is seen to consist of flattened epithelial cells lined bj^ a 

 thin chitinous intinia, but no basement membrane can be distinguished. 

 The usual muscle layers are present, but are so fine as to be distinguished 

 only with consideral)le difficulty. At rest and empty, as it is seen in 

 sections, the wall shows a number of small convolutions. 



The " mandibles " of Endedein 

 On either side of the pumping pharynx, where the posterior arms of 

 the buccal plate fuse with its lateral walls, lie two triangular chitinous 



