36 
Argas persicus 
this foramen by a thin articular cuticle which forms a deep iuvaginated 
fold along the dorsal, margin, to allow free movement in the act of 
depression of the capitulum. The basis capituli is freely articulated 
with the prosoma and is capable of a considerable degree of movement 
in the sagittal plane. 
The walls of the basis capituli are formed of stout and resistant 
chitin which affords a firm attachment for certain muscles. The ventral 
wall of the basis capituli is prolonged anteriorly, in its median portion, 
to form the hypostome {h.), on either side of which the basal articles of 
the palps (p.i.) are inserted. On the dorsal side, the basis capituli 
narrows down to form what is at first a sheath common to both 
chelicerae ; this bifurcates almost immediately and is prolonged forwards 
to foi;m a pair of tubular sheaths (s.ch.), through which the chelicerae 
protrude, the whole arrangement resembling a short pair of trousers. 
The opening of the mouth lies concealed between the basal portions of 
the hypostome and the chelicerae, and is hidden from lateral view by 
the bases of the palps. On the ventral surface of the basis capituli, a 
number of hairs are borne. Of these, four are conspicuous, viz. a pair 
of post-hypostomal hairs (PI. IV, fig. 8, p.h.h) situated near the median 
line, immediately behind the base of the hypostome, and a pair of post- 
palpal hairs ( p.p.h.) which are situated a short distance behind the bases 
of the palps. A few stumpy hairs occupy the lateral portions of the 
ventral surface. The dorsal surface is scattered with numerous, 
anteriorly directed hair.s, which, together with those on the palps and 
the roof of the camerostome, serve to exclude foi-eign matter from the 
camerostomal depression (see PI. Ill, fig. 7). 
The Chelicerae. 
Plate IV, figs. 8, 9 and 10; Plate VI, figs. 22-2.5. 
The chelicerae, which form the first pair of appendages, are situated 
above the oral opening and buccal cavity. Their free, distal extremities 
protrude anteriorly for a considerable distance in front of the actual 
opening of the mouth and their posterior ends extend backwards, 
through the basis capituli and the capitular foramen, into the body 
cavity. Each consists of a stout, more or less cylindrical shaft (sh.ch.), 
the proximal half of which is dilated {b.ch.). The distal extremity of 
the shaft bears a terminal, modified chelate digit (d.ch.). 
The chelicerae slide freely, backwards and forwards, within the 
sheaths (s.ch.) formed by the basis capituli. Their shafts are connected 
