38 
Argas jwrsims 
of the chelicerae is effected, not by direct muscular action, but by a 
general contraction of the dorso-ventral body-muscles. By the pressure 
thus created within the body cavity, the chelicerae are thrust out 
through their sheaths until further movement is stopped by the 
expanded basal portions coming into contact with that portion of 
the basis capituli which forms a common sheath (see Text-fig. 2) round 
the two cheliceral shafts. 
The cavity of the expanded proximal portion of the cheliceral shaft 
is completely filled by the flexor and extensor muscles which operate the 
digit. The tendons of these muscles pass forwards through the narrow 
distal portion of the shaft to their terminations at their respective places 
of insertion into the digit, each being furnished with a guide, in the 
form of a narrow chitinous canal which runs in the thickened wall of 
the shaft. The guiding canal of the flexor tendon (PI. VI, figs. 22-24, 
t.f. d.) is situated in the lateral portion of the wall of the shaft and 
extends from the commencement of the contracted portion of the latter, 
almost to the articulation of the digit. This is clearly shown in the 
transverse sections of the capitulum (see PI. V, figs. 11-15). The 
extensor tendon {t. e. d.), which is the more slender of the two, is situated 
tow’ards the internal and dorsal portion of the shaft. Its guiding canal 
is neither so well-developed nor so extensive as that of the flexor tendon 
and is not shown in the sections on PI. V. 
The digit of the chelicera (PI. IV, figs. 8, 9 and lU, d. ch. ; PI. VI, 
figs. 22-25), which forms the terminal portion of the appendage, has 
received considerable attention in the past, chiefly as a result of the 
idea that the forms of its component parts are of value for the purpose 
of specific determination. The structure is, however, subject to no 
inconsiderable amount of individual variation, and, moreover, its pre¬ 
paration for such an examination involves not only its removal from the 
specimen, but, on account of its minute size, a troublesome process of 
mounting. For both reasons therefore an account of the digit is best 
omitted from specific descriptions. 
In the fact that it is comprised of two mobile articles, the digit of 
the chelicera conforms to the chelate type generally found in the 
Arachnida-, but, in accordance with the mode of life followed by the 
tick, the usual pincer-like organ has been modified to form a cutting- 
organ, pure and simple. 
The larger internal article {i. a.) has the form of an irregular 
triangular pyramid, the apex of which terminates in a laterally directed 
cusp. It is attached by its expanded base to the extremity of the 
