XI 



PHYLUM ARTHROPODA 



649 



At the extremity of the abdomen is the spinning apparatus or 

 arachnidium (Fig. 561, arach.). This consists of four or six eleva- 

 tions, the spinnerets, usually 

 jointed, probably derived from 

 embryonic rudiments of abdo- 

 minal appendages. On the sur- 

 faces of these open the numerous 

 fine ducts of the spinning glands 

 (sp. glds.) } secreting the material 

 of which the spider's web is com- 

 posed. The fine threads of viscid 

 secretion issuing from the ducts 

 harden on exposure to the air, 

 and are worked up into the web 

 by means of the posterior legs. 

 There are six or eight eyes on 

 the carapace. 



In the spider-like Phalangida, 

 or " Harvestmen," the cephalo- 

 thorax is not constricted off FIQ - 

 from the abdomen. The chelicerae 

 are chelate, the pedipalpi short and leg-like, the legs long and 



slender. 



In the Acarida, 

 or Mites and Ticks 

 (Figs. 556 and 557), 

 the distinction into 

 regions is no longer 

 recognisable. The 

 form of the mouth- 

 parts varies some- 

 what in the different 

 families. Some- 

 times the basal 

 portions of the 

 pedipalpi form a 

 sucking proboscis 

 enclosing the stylet- 

 like chelicerae, 

 modified to form 

 piercing organs ; 

 sometimes these 

 appendages are 

 claw-like or chelate. 

 The legs vary 

 somewhat in shape 



FIG. 557. Water mite (Trombidium fuliginosum), female. ; Jift^ 



chel. chelicerae ; ped. pedipalpi. (After Leuckart.) * n t H 6 Clllierent 



