THE Hoa Louse 683 
by a strand of tissue. Anteriorly it appears to lie free between the 
elements, while just behind the haustellum it lies within the canal of the 
dorsal part of the ventral element. This duct was seen and figured by 
Stevenson (1905:13), but its function was not recognized until Harrison 
(1916 b:209) carried out his investigation of the mouth parts. 
When the piercers leave the sheath at the junction of the buccal plate 
with the pumping pharynx, they bend at an obtuse angle and pass forward 
in the groove of the buccal plate beneath the pumping pharyngeal tube 
to the mouth opening (Plate LXI, 1-4). 
Musculature of the mouth parts ! 
In the region of the rami the sheath is no longer a structure distinct 
from its contents, and both sheath and contents are controlled by one 
set of protractor muscles (Plate LX,6). These originateas slender strands 
in the posterior end of the sheath, where the free ends of the rami are 
imbedded in its wall. They pass forward along the ventro-lateral borders 
of the sheath and are inserted in the lateral borders of the ventral plate 
(Plate LX, 6). The individual strands vary in length, so that, if they be 
detached from their origin and pulled away from the sheath, they resemble 
the extended dorsal fin of a fish. The longest strands extend to the 
anterior border of the plate. The contraction of these muscles bends 
back the ventral plate and telescopes the hinder part of the sheath into 
the front part, so that the piercers are pushed out of the head. 
The retraction of the piercers and the sheath to their resting position 
is brought about by two sets of retractor muscles, a lateral and a posterior. 
The lateral retractors consist of two muscles originating in the wall of 
the head and inserted in the lateral wall of the sheath in the region of 
the anterior border of the ventral plate. The dorsal lateral retractor 
originates in the dorso-lateral posterior angle of the head and passes 
obliquely downward and forward between the bands of the tendon muscle 
and the brain to its insertion in the sheath. The ventral lateral retractor 
is considerably shorter than the dorsal, and originates in the latero- 
ventral wall of the head alongside of the ventral retractor of the pharynx, 
whence it passes forward to its insertion in the sheath (Plate LX, 6). The 
posterior retractors are two large muscles lying on either side of the end 
of the sheath almost in the neck, two muscles lying under its ventral 
surface, and two lying on its dorsal surface. Hach of the first has a 
