490 



ARACHNID!. 



fused with the thorax. The oral apparatus is adapted for biting or 

 piercing and sucking,. Respiration, as a rule, by means of trachece. 



The body of the Acarina is generally small and possesses a 

 stout and unsegmented form. The head, thorax, and abdomen are 

 fused into a common mass (fig. 379). The form of the oral 

 apparatus varies exceedingly, and may be adapted either for biting 

 or for piercing and sucking. The chelicene are accordingly some- 

 times retractile styles, and are sometimes furnished with claws or 

 chela?. In the first case, the bases of the pedipalpi form a sheath 

 which surrounds the styliform chelicera3 and serves as a suctorial 

 rostrum, while the peripheral part of the pedipalpus or palp frequently 



projects laterally, and ends 

 with a claw or chela. The 

 structure of the four pairs of 

 legs is not less various, in- 

 asmuch as they may serve for 

 crawling, attachment, running 

 and swimming. They usually 

 end with two claws, sometimes 

 in parasitic forms with stalked 

 suctorial discs. 



The nervous system is re- \ 

 duced to a common ganglionic 

 mass representing the brain 

 and ventral cord. Eyes may 

 be absent or may be present, 

 as one or two pairs of simple 

 eyes. 



The alimentary canal is 

 frequently provided with sali- 

 vary glands, and gives off on 

 either side a number of blind saccular diverticula which may be 

 forked (fig. 380). 



Heart and blood vessels are invariably absent, but respiratory 

 organs are frequently present in the form of trachea, which arise 



1'ordre des Acariens en general et les families dcs Trombidies, Hydrachnes en 

 part," Ann. des Sc. j\'at., II. Ser., Tom. I. and II. H. Nicolet, " Histoire 

 naturelle des Acariens, etc. Oribatides." Archives du inusee cThist. not.. 

 Tom VII. O. Fiirstenberg, "Die Kratzmilbcn des Menschen und der Thicre," 

 Leipzig, 1861. Al. Pagenstecher, " Beitrage ztir Anatomic der Milben," I. and 

 I.I., Leipzig, 1860-1861. E. Claparede, " Studien an Acariden," Ze.it sclir. fur 

 miss. Zool., Tom XVIIL, 1868. P. Megnin, "Les parasites et les maladies 

 aprasitaires, 1880. 



Fio. 380 Anatomy of Ixodes Jt'clnus (after Al. 

 Pagenstecher). G, Brain ; SpD, salivary gland ; 

 Dg, ducts of salivary gland ; D, diverticula of 

 intestine ; A, anus ; N, urinary organ ; Tr, 

 bundles of trachea? ; St. stigma. 



