AEACIINIDA. 



[ GO ] ARACHNOID MExMBRANE. 



damp places beneath moss, upon sand of 

 caves, &c. 



Fam. 7. Trombidiua (Harvest-mites). 

 Palpi with the last joint obtuse, the 

 second joint very large ; the last but 

 one (penultimate) resembhng an incur- 

 vated claw ; feet cursorial, terminated by 

 two claws. 



Fam. 8. Hypoderidae (Subcutaneous 

 mites). Oval-oblong, almost transparent 

 sacs ; the anterior and posterior pairs of 

 legs widely apart ; tarsi terminated by two 

 fine hairs. 



The species are found in the subcu- 

 taneous tissue of birds. 



Fam. 9. Phytoptidte (Gall-mites). 

 Legs four, the hinder pairs replaced by 

 tubercles or hairs. 



In the galls and buds of plants. 

 Order 7. Tardigeada (Colopoda) ; Water- 

 bears. Legs rudimentar}^, very short, 

 conical, indistinctly three-jointed, and 

 with three or four claws ; abdomen not 

 distinct from the thorax. (Aquatic.) 

 Order 8. Pycnogonida (Polygonopoda) ; 

 Crab-spiders. Cephalothorax forming a 

 4-jointed body ; abdomen rudimentary 

 (small and conical) ; legs as long as or 

 longer than the body. 



Sluggish marine animals, living under 

 stones, upon marine plants, or parasitic 

 upon fish and Crustacea. 

 BiBL. Treviranus, Bau der Arachn. ; 

 id. Vermischte Se/u-{ft. ^-c. Bd. 1, 1816; 

 Dufour, An. d. So. phi/sicj. d. Bru.v., iv.-vi. ; 

 Walckenaer, Hist. nat. d. Ins. Apt., i.-iii. ; 

 Siebold & Stannius, Lehrb. d. vergl. An. i. ; 

 Blackwall, Brit. Spiders {Ray Soc.) 1861-4, 

 and Linn. Tr. xvi. ; Blanchard, Ann. A\ H. 

 18-"i0, vi. 67 ; and 18-j2, x. loO ; Newport, 

 Phil. Tr. 1843 ; Koch, Uebersicht 8fc., and 

 Arachnid. (534 pis.) ; Claparede, Vevol. i^'c. 

 1863 ; id. Circulation, ^-c, Ann. N. H. 1865, 

 XV. p. 16; Cambridge, Encxjl. Brit. 1875, 

 ii ; Jones, An. Kingd. 1870 ; Gegeubaur, 

 Vergl. Anat. 1878 ; Walker, BrU. Spiders 

 {Eay Soc. ) ; Murrav, Econ. Ent. p. 33. 



ARACHXID'IUM, Hincks.— A genus of 

 Ctenostomatous Polyzoa (Bryozoa), fam. 

 Arachnidiidse. 



Three British species. 

 BiBL. Hincks, Poh/zoa, 1880, p. 508. 

 ARACHNOID MEMBRANE {Tunica 

 arachnoidea). — Is a delicate transparent 

 membrane, lying between the cranial dura 

 mater and the brain, and extending between 

 the spinal cord and its dura mater so as to 

 envelope these nervous centres. It does not 



dip between the convolutions of the brain, 

 but enters and lines its ventricles. Its outer 

 surface is covered by a delicate epithelial 

 layer ; its inner surface is smooth, but not 

 covered with epithelium. It is reflected 

 upon the surface of the dura mater as an 

 epithelial layer only. It consists principally 

 of reticulated bundles of connective tissue, 

 with fibres of elastic tissue coiling around 

 or pursuing a rectihnear course through 

 them. In some parts the fibrillse of the 

 former run parallel without forming bundles, 

 and contain, as do the bundles, round, elon- 

 gated, or spindle-shaped nuclei. In others, 

 connective tissue of a rather homogeneous 

 appearance here and there forms a coat to the 

 bundles, or is situated between them. 



F"ig. 29 represents two bundles of the 

 connective tissue of the human arachnoid, 

 after the addition of acetic acid, showing 

 the fibres of elastic tissue. 



Fig. 29. 



Connective and elastic tissue of arachnoid, after treat- 

 ment with acetic acid. Magnified 350 diameters. 



BiBL. Kolliker, Mik. An. ii. : Henle, All- 

 gem. Anat. 



ARACHNOIDIS'CUS, Bailey (Hemi- 

 ptychus, Ehr.). — A genus of Diatomaceae. 



Char. Fiustules adherent, disk-shaped; 



