536 THE INVERTEBRATA 



by stigmata, the position of which varies in the main divisions of the 

 group. In the Notostigmata there are four pairs of dorsal stigmata in 

 the first four opisthosomatic segments; in the Cryptostigmata, four 

 pairs of stigmata at the bases of the four walking legs ; in the rest there 



^xhc-sh. 



Fig. 368. Argas persicus, S. Median longitudinal section showing the pro- 

 boscis, alimentary canal and reproductive systems. Altered from Nuttall. 

 The chelicerae are seen within the cheliceral sheath (chc.sh.). They are thrust 

 forward by the contraction of dorsoventral body muscles (m.r.ch.) and cut their 

 way into the host by the digits (chc.d.) which are moved by their flexor muscles 

 {m.f.d.). The barbed hooks of the hypostome {hyp.) are thrust into the wound 

 and keep the tick in place, an. anus ; b.cap. basis capituli ; brn. concentration 

 of nervous system ; cav.sh. cavity of cheliceral sheath ; coe. caecum of the 

 stomach; end. endosternite ; h. heart; M. mouth cavity; oe. oesophagus; ph. 

 pharynx with radiating muscles; r.s. rectal sac; st. stomach; sal.d., sal.gl. 

 salivary duct and gland ; ts. testis ; vd. vas deferens ; w.gl. white (accessory) gland. 



is a single pair of stigmata in varying positions, in front of the chelicerae 

 (Prostigmata), between the chelicerae and pedipalps (Stomatostig- 

 mata), between the pedipalps and ist walking legs (Heterostigmata), 

 the 2nd and 3rd legs (Parastigmata), the 3rd and 4th (Mesostigmata), 



