ARACHNIDA. 



[ 58 ] ARACHNOID MEMBRANE. 



nerally terminated by a vesicle or adhesive 

 acetabulum and claws. 



Gen. Acarus{Tyroglyphus), Trichodac- 

 tylus, Psoroptes, Sarcoptes, Demodex. 



Fam. 2. Oribatea (Notaspidea). Body 

 covered by a hard horny envelope ; man- 

 dibles chelate; palpi fusiform, five-jointed; 

 feet fvu'nished with claws, but no vesicle 

 nor acetabulum. 



Gen. Oribata (Notaspis), Hopophora, 

 Damceus, Zetes, Pelops. 



Most of the species live in mosses at the 

 roots of trees ; in some the body is sur- 

 rounded by a projecting lamella on each 

 side. 



Fam. 3. Ixodea. 



Gen. Ixodes. 



Fam. 4. Gamasea. Palpi free, filiform; 

 mandibles chelate; feet with two claws 

 and a caruncle, or a lobed membranous 

 appendage ; ocelli none or indistinct. 

 (Generally parasitic.) 



Gen. Dermanyssus, Uropoda, Caris, 

 Gamosus. 



Fam. 5. Ilydrachnea. Palpi with the 

 last joint unguiculate or spinous ; two or 

 four distinct ocelli ; coxae broad, legs ge- 

 nerally ciliated, natatory, the posterior 

 longest. (Aquatic.) 



Gen. Limnochares, Eylais, Ilydrachna, 

 Atax, Arrenurus, Diplodontus. 



Fam. 6. Bdellea. Palpi antenniform ; 

 mandibles terminating in claws or pincers; 

 rostrum resembling an elongated head ; 

 body generally divided between the second 

 and third pairs of feet by a transverse fur- 

 row or stricture ; (abdomen many -jointed). 



Gen. Bdella. 



The species consist of minute animals, 

 more or less soft, variously coloured, and 

 living in damp places beneath moss, upon 

 sand of caves, &c. 



Fam. 7- Trombidina. Palpi with the 

 last joint obtuse, the second joint very 

 large; the last but one (penultimate) 

 resembling an incurvated claw ; feet cur- 

 sorial, terminated by two claws. 



Gen. Anystis, Cheyletus, Tetranychus, 

 Megamerus, P achy gnat hus, Raphignathus, 

 Smarts, Erythrceus, Trombidium{heptus). 

 Order 4. PHALANGiTA(Opilionina). Cepha- 

 lothorax conjoined with the abdomen ; 

 abdomen annulate or transversely plicate ; 

 palpi simple, filiform; mandibles didac- 

 tyle ; feet elongate, terminated by a single 

 claw. 



Gen. Trogulus, Phalangium, Eusarciis, 

 Gonyleptes, 



Order 5. Pseudoscorpiones. Cephalo- 

 thorax conjoined with abdomen; abdomen 

 annulate ; palj)i large, chelate. 

 Gen. Obisium, Chelifer. 

 Order 6. Solifug^. Cephalothorax distinct 

 from the abdomen; abdomen annulate; 

 palpi filiform, extended, equalhng the feet 

 in length. 

 Gen. Galeodes. 

 Order 7- Pedipalpi. Abdomen jointed, di- 

 stinct from cephalothorax ; palpi large, 

 resembling feet, chelate at the apex ; ])ul- 

 monary sacs, but no tracheae; stigmata 

 four or eight. 



Gen. Thelyphonus, PJirynus, Scorpio, An- 

 droctonus, Buthus. 



Order 8. Araneida. Abdomen not jointed^ 

 nor cephalothorax separated by a constric- 

 tion ; respiratory organs consisting of tra- 

 cheae and pulmonary sacs. 

 Gen. Aranea (Tegenaria), Epeira, &c. 

 BiBL. Treviranus, Ueber den inner. Bau 

 der Arachniden ; id. Vermischte Schriften, 

 Sfc. Bd. 1, 1816; Dufour, Ann. d. Sc.pliy- 

 siq, de Bruxelles, iv.-vi.; Walckenaer, Hist. 

 nat. d. Ins. Apt., i.-iii. ; Van der Hoeven, 

 Handb. der Zoologie, i.; Siebold & StanniuSj 

 Lehrb. d. Vergleich. Anat. i,; Owen, Hunter. 

 Lectures, i.; Blackwall, Ann. N. Hist. xv. Apr. 

 1845, and Linn. Trans, xvi ; Blanchard, Ann. 

 N. H. 1850, vi. 67 ; and 1852, x. 150; New- 

 port, Phil. Trans. 1843; Koch, Deutschlands 

 Crustac St'c 



ARACHNOID MEMBRANE {Tunica 

 arachnoidea) — Is a delicate transparent mem- 

 brane, lying between the cranial dura mater 

 and the brain, and extending between the 

 spinal cord and its dm'a mater so as to enve- 

 lope these nervous centres. It does not dip 

 between the convolutions of the brain, but 

 enters and lines its ventricles. Its outer sur- 

 face is covered by a delicate epithelial layer; 

 its inner surface is smooth, but not covered 

 with epithelium. It is reflected upon the 

 sm'face of the dura mater as an epithelial 

 layer only. It consists principally of reticu- 

 lated bundles of areolar (cellular) tissue, with 

 fibres of elastic tissue coiling around or pur- 

 suing a rectilinear course through them. In 

 some parts the fibrillae of the former run par- 

 allel without forming bundles, and contain, 

 as do the bundles, round, elongated, or 

 spindle-shaped nuclei. In others, areolar 

 tissue of a rather homogeneous appearance 

 here and there forms a coat to the bundles, 

 or is situated between them. 



Fig. 31 represents two bundles of the areo- 

 lar tissue of the human arachnoid, after the 



