464 THE INVERTEBRATA 



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 

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



Fig, 344. Argas persicus, (^. 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 

 (jn.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 ; ^/z. 

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

 salivary duct and gland ;f^. testis ;z;^.vas deferens ; w.gl. white (accessory) gland. 



(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), 

 and behind the 4th legs (Metastigmata) If we regard the opistho- 

 somatic position of the breathing organs as primitive it is difficult to 



