L. E. Robinson and J. Davidson 387 
with the integumental grooves of the Ixodicl ticks and also, to a certain 
extent, the markings on the scutum of ornate species. 
The most cursory examination of the arrangement of the discs on 
the dorsum of Argas persicus enables the observer to recognise the 
relationship between certain rows of discs and the corresponding grooves 
of the Ixodid ticks (see Text-fig. 2). A postero-median row of discs 
represents the postero-median groove and a pair of adjacent shorter 
rows occupy the position of the accessory grooves. In order to clearly 
recognise the homologies of the remaining discs with the integumental 
grooves of the Ixodidae, some knowledge of their relations with the 
underlying muscles is necessary and it will be more convenient, there¬ 
fore, to consider the distribution of the discs at the same time as their 
corresponding muscles are under review. 
For convenience in description, the musculature may be classified 
as follows : 
(a) The Body-Muscles. 
(b) The Muscles of the Appendages. 
(c) The Muscles of the Alimentary Canal. 
(d) The Muscles of the Genitalia. 
(a) The Body-Muscles. 
With certain exceptions to be mentioned later, the body-muscles are 
columnar strands, which pass directly in a vertical direction, or in some 
cases with a slight degree of inclination, through the body-cavity, their 
upper and lower extremities being inserted into the dorsal and the 
ventral integument respectively. In their course through the body- 
cavity, they pass through the notches formed by the alimentary coeca 
and their terminal sacculations. They are disposed in a regular manner, 
many of them forming more or less regular rows of variable length and 
as their upper extremities, and in most cases the lower extremities also, 
are attached to the internal surfaces of the integumental discs, the 
arrangement of the latter is determined absolutely by the body 
musculature. By their contractions, the dorso-ventral body-muscles 
approximate the body walls, thus bringing pressure to bear on the 
contents of the body-cavity. In this manner, they serve many purposes, 
among which may be mentioned the following :— 
They facilitate the circulation of the coelomic fluid; they assist in 
forcing the ingested blood into the marginal terminations of the alimen¬ 
tary coeca; they serve to expel waste air from the tracheal system 
