L. E. Robinson and J. Davidson 
241 
posterior to the rectal opening. It is formed of an epitlielium of more 
or less columnar cells which are apparently similar in nature to the 
hypodermal cells. At the anal orifice this epithelium merges gradually 
into the general hypodermis of the ventral body-wall ; at the upper 
extremity of the anal canal it extends for a short distance into the 
rectal sac. The anal canal is lined within by a very thin chitinous 
cuticle which is continuous with the general integumental chitin and 
extends as far as the opening into the rectal sac, where it tliins out 
and disappears. 
The structure of the anus has already received attention in Part I 
of this paper (p. 32), but one or two details of internal structure should 
be referred to. 
The muscles of the anus consist of a single pair of obliquely inclined 
bands which extend from the upper part of the anal canal to the lateral 
margins of the anal valves. By their contraction, the lips of the anal 
aperture are everted, and the expulsion of the excreta is apparently 
brought about by the contraction of the dorso-ventral body muscles. 
The anal muscles, which Nordenskibld^ described in Ixodes ricinus as 
extending from the anal ring to the dorsal integument, are absent, and 
the large muscles which appear in his figure are evidently the foremost 
columns of the series of postero-median dorso-ventral body muscles. 
The true anal muscles are, however, represented in his figure, and appear 
to be identical, in their relations to the anal canal and the anal valves, 
with those of Argas persicas. 
The blood-vasculak system. 
Though more primitive than the circulatory system of some of the 
Arachnida {Scorpionidae and Araneidae), the blood-vascular system of 
Argas persiciis does not appear to be quite so degenerate as the obser¬ 
vations of some of the workers on the anatomy of the Ixodoidea would 
lead one to suppose. It is, however, generally known that in Arthropoda 
with a highly developed tracheal system, the peripheral portions of the 
blood-vascular system are correspondingly reduced, and in some cases 
even the heart may disappear; while in the case of Arachnida which 
show a concentration of the respiratory system, in the form of “ book- 
gills,” the heart and vascular arrangements are more complex and 
well-developed. 
1 Nordeuskiold, E. (1908), p. G17 and PL 27, fig. 13. 
