L. E. Robinson and J. Davidson 
247 
within the body-cavity and to communicate with the exterior through 
the cleft. The section through the crescentic plate shows it to consist 
of a deep layer of chitinous cuticle to which the hypodermis is apposed, 
and from the outer surface of which large numbers of slender chitinous 
processes extend to the surface of the crescentic plate where they appear 
to fuse together by their expanded extremities to form a continuous 
outer plane of thin chitin. Close examination shows, however, that 
between the outer extremities of these slender processes, an occasional 
hiatus appears in the section, which occurs where the minute pores, 
with which the surface of the crescentic plate is scattered, are cut across. 
If, now, these structures are compared with those shown in a section 
through the spiracular plate of Haemaphysalis punctata, it is evident 
that the postero-internal area (mac.) corresponds to what was described 
in the paper cited, as the macula, which, in all the Ixodid ticks, occupies 
an isolated position in or near the middle of the spiracular plate, being 
completely cut off from the general body cuticle by the a7'ea porosa 
(a.por.). The latter is represented in a reduced degree by the perforate 
ci'escentic area of the spiracle of A. pei'sicus, in which case it falls far 
short of completely surrounding the macula. The slender chitinous 
processes are homologous with \h.Qpedicels of H. punctata. The pyriform 
spaces and the internal pores of the latter species are absent in A. persicus. 
The pores of the area porosa establish a communication between the 
interpedicellai' air spaces and the exterior, and the interpedicellar air 
spaces are continuous with the cavity of the atrium. 
The atrium is a somewhat elongated cavity the lumen of which is 
contracted about its middle. It communicates with the exterior through 
the ostium and also, by way of the interpedicellar air spaces, through the 
pores of the area porosa. Its walls are composed of two layers—an inner 
hypodeimxis and an outer chitinous cuticle —both of which are continuous 
with the general integument of the body. The hypodermal layer pos¬ 
sesses larger and more numerous nuclei than that of the general 
integument, but is otherwise identical in structure. The constriction 
of the lumen of the atrium, mentioned above, is brought about by the 
fact that a limited portion of the atrial wall, the cuticle of which is 
thicker than the rest, bulges out into the cavity; this projection 
apparently serving to close the atrium by being squeezed against the 
opposite wall of the cavity, an operation which is possibly effected by 
the contraction of the dorso-ventral body muscles. Immediately internal 
to this structure, the cuticular lining of the atrium shows a series of fine 
parallel ridges (see Text-fig. 6) which, though they do not receive 
