THE Hoc 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 résumé of the literature of the heart, 
beginning with the work of Wed! (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 be 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 
