264 ENTOMOLOGY FOR MEDICAL OFFICERS 



of the legs are modified to act as " jaws " for helping to squash 

 prey, or for closing the vestibule of the mouth. The alimen- 

 tary canal is not convoluted, but the digestive and absorbent 

 surface is increased by numerous paired diverticula, or gastric 

 pouches. The anus is ventral and subterminal. In the ter- 

 restrial Arachnida excretory Malpighian tubules open into 

 the hinder part of the gut as in insects, but additional excre- 

 tory glands opening on the coxae of the legs may also be 

 present. 



Some Arachnida are viviparous, but the majority lay 

 eggs. The reproductive organs usually open ventrally at the 

 anterior end of the abdomen. As a rule the young resembles 

 the parents, but in the ticks and their kind the young differs 

 from the adult in having only three pairs of legs, and does not 

 acquire the fourth pair until after the first moult. 



Existing Arachnida are grouped in the following eleven 

 Orders, only four of which, namely, the Acarina, Scorpionidea, 

 Araneida, and Solifugce, need be further considered. 



Order i : Pycnogonida. Marine Arachnida with a slight 

 superficial resemblance to Spiders without an abdomen. 

 They are found at all depths, from the tide-marks to the 

 abysses of the ocean. 



Order 2 : Xiphosura, or King-crabs. Large marine 

 Arachnida with the cephalothorax enclosed in a horseshoe- 

 shaped carapace, a roughly pentagonal abdomen carrying 

 leaf-like appendages to which the gill-books are attached, and 

 a long post-anal spine. They are found in muddy shoal 

 water on the Atlantic coast of America and on the shores of 

 Southern and South-eastern Asia. 



Order 3 : Scorpionidea, p. 294. 



Order 4 : Pedipalpi. Arachnida which have a general 

 resemblance to Scorpions, but differ in having the pedipalps 

 not chelate, or only imperfectly so, the first pair of legs 

 antenna-like, the post-abdomen or " tail " either very much 

 attenuated or altogether absent, and no post-anal spine. 

 They are predaceous, chiefly on insects, and are found in the 

 Oriental, Ethiopian, and Neotropical regions. 



Order 5 : Araneida; Spiders, p. 298. 



Order 6 : Palpigradi Minute Arachnida with some 

 resemblance to tiny spiders, but differing, inter alia, in having 



