The Hog Louse 667 



again called attention to the similarity of the heart in Mallophaga and 

 Siphunculata (Anoplura), and referred to Fulmek's (1905) work on 

 Mallophaga, in which there is a short resume of the literature of the heart, 

 beginning with the work of Wedl (1855), who first distinguished in the 

 dorsal vessel a posterior, specially contractile part — the true heart — 

 and an anterior, more vessel-like part — the aorta. 



In the hog louse the heart lies in the two posterior abdominal segments, 

 between the halves of the dorsal muscle plate, and is attached to the 

 dorsal wall on either side of the median line by two delicate septa. It 

 is oblong-ovate, measuring approximately 0.38 millimeter in length 

 and 0.075 millimeter in breadth, and has two lateral indentations on 

 either side which give it a three-chambered appearance (Plate LIX, 6). 

 Attached to the lateral and ventral surfaces are three pairs of wing muscles 

 which pass directly laterad under the two halves of the dorsal muscle 

 plate and are inserted in the lateral body wall toward the ventral surface. 

 To the central wing muscles on either side is attached a group of six peri- 

 cardial cells similar to those described by Fulmek (1905:620) in Nirmus 

 sp. In gross dissections the ostia cannot be clearly seen, but sections 

 show three pairs, lateral in position. Anteriorly the heart leads into the 

 aorta, which lies free throughout most of its length in the body cavity 

 and passes cephalad entering the head alongside the esophagus. Its 

 width varies from 0.03 millimeter at the posterior end to 0.02 millimeter 

 at the anterior end. In some few cases it seemed swollen to a bulb in the 

 region of segments 6 and 5, but we did not find this to Ije a constant 

 character. 



The wall of the heart is very thin, and in section it is seen to be of 

 uneven thickness (Plate LIX, 7). Its histological elements appear to be 

 mostly muscular, and, while nuclei are visible, they resemble those of 

 the sarcolemma rather than those of a true epithelium. Where the wall 

 is slightly contracted, it has a false appearance of being toothed. Where 

 the heart passes into the aorta there is a succession of six pairs of valve- 

 like structures extending from opposite walls of the aorta into its lumen 

 and almost meeting on the median line. Sections showed no definite 

 structure that would reveal the true histological nature of these. 



The blood is a colorless fluid and its cells can be seen singly and in 

 groups scattered throughout the heart and the aorta. They are definite 

 round cells with a well-defined central nucleus, and do not appear to be 



