ARACHNIDA. 529 



The Arachnida are classified as follows : 



A. Tracheate groups. 



1 . Linguatulina : parasitic, worm-like, elongated, body ringed ; Pentastomum. 



2. Acarina : abdomen united to cephalothorax ; oral appendages adapted to 

 biting, or piercing and sucking ; mostly tracheate ; minute. Demodex, Sarcoptes, 

 Tyroglyphus, Hydrachna, &c. 



3. Tardigrada : no abdomen ; four pairs of short limbs ; oral appendages 

 adapted for piercing and sucking. Hermaphrodite. Minute, aquatic. Arctiscon^ 

 Macrobiotus, &c. 



4. Araneidae : abdomen soft, unsegmented ; fakes with a poison-gland ; 4-6 

 spinnerets ; 2-4 lung-sacs. Mygale, Cteniza, Tegenaria, &c. 



5. Phalangidae : abdomen as a rule with six somites, broad and applied to 

 cephalothorax ; falces with a didactyle claw or chela ; limbs long and slender ; 

 tracheate. Phalangium, &c. 



6. Pedipalpi: abdomen with 11-12 somites; falces clawed; pedipalpi clawed 

 (Phrynus) or ch elate ( Thelyphonus) ; first pair of limbs slender and elongate; four 

 lung-sacs. Phrynus, Thelyphonus. 



7. Scorpionidae : abdomen composed of a seven-segmented mesosoma and 

 five-segmented metasoma, with telson armed with a poison-gland; both oral 

 appendages chelate ; second pair large ; four lung-sacs. Androctonus, Buthus, 

 Scorpio, &c. , 



8. Pseudo-scorpionidae : abdomen broad and flat, with 10 n somites; pedi- 

 palpi chelate ; tracheate. Chelifer, Obisium, Chthonius, &c. 



9. Solifugae : head and thorax separate ; thorax of three separate somites ; 

 abdomen with nine somites ; falces chelate ; palpi limb-like, tracheate. Galeodes 

 {Solpuga). 



B. Branchiate groups =Poecilopoda of Wallcott. 



1. Xiphosura \ cephalothorax shield-shaped ; meso- and meta-soma fused ; first 

 pair of oral appendages chelate ; coxae of the five pairs of cephalothoracic limbs 

 masticatory; six pairs of mesosomic limbs; a telson. Limulus, Belinurus (fossil). 



2. Eurypterina s. Merostomata : extinct ; abdomen with twelve somites and a 

 broad telson ; first pair of oral appendages sometimes non-chelate ; four pairs of 

 cephalothoracic limbs surround mouth ; no abdominal limbs. Eurypterus, Ptery- 

 gotus, Hemiaspis, &c. 



3. Trilobita\ extinct; a cephalothorax; a segmented mesosoma and un- 

 segmented metasoma (pygidium), both bearing limbs ; no prae-orally placed 

 appendages, but four pair of cephalothoracic limbs surround the mouth. Catymene, 

 Phacops, Asaphus, &c. 



'Arachnida,' P. Cambridge, Encyclopaedia Britannica (ed. ix.), ii. 1875; 

 Packard, Study of Insects, Salem, 1872; Murray, Economic Entomology, S. 

 Kensington Handbooks, 1877; Blanchard, ' Arachnides,' L'organisation du Regne 

 animal, Paris, 1860; Zoological position of Class, Kingsley, Q. J. M. xxv. p. 556. 



Linguatulina, P. J. Van Beneden, A. Sc. N. (3), xi. 1849; Leuckart, Bau der 

 Pentastomen, Leipzig, 1860. 



Acarina, see pp. 496-7, ante. Classification, cf. Michael, Oribatidae, Ray Soc. i. 

 cap. 4; Demodex, Me'gnin, Journal de 1'Anat. et Physiol. xii. 1877; Sarcoptidae, 



M m 



