ARACHNIDA. 



L 57 ] 



ARACHNIDA. 



cutaneous plate, at the posterior margin of 

 which the respiratoiy fissure exists. Behind 

 these fissures there are two other openings, 

 the orifices of a tracheary system which 

 does not difi^er materially from that of such 

 as have tracheae only. 



Nervous System. — Varies in degree of com- 

 plexity. In its simplest form, it exists as a 

 single oesophageal ganglion, sending off ra- 

 (hating branches; and in its most compound 

 forms, it presents a large cephalo-thoracic bi- 

 lobed ganglion, and one or two ventral gan- 

 glial chains or cords. 



The primitive nervous fibres and ganglion- 

 cells are very small and delicate. 



Spinning organs . — These organs, by means 

 of which the Araneidae form their webs, are 

 of great interest. The external organs con- 

 sist of three or rarely two pairs of cones or 

 conical papillae, or spinnerets, placed at the 

 end of the abdomen, below the anus : they 

 are som.ewhat flattened at the summit, and, 

 usually, the middle pair consists of two joints 

 and the anterior and posterior pairs of three 

 joints. The sides of the cones are covered 

 with hairs, and on the summits are a number 

 of delicate horny spinning tubes, at first sight 

 closely resembling hairs; these form con- 

 tinuations of the spinning vessels. Some- 

 times, however, the lower portions of the sides 

 of the cones are furnished with spinning- 

 tubes, the remainder being covered with 

 hairs. Each spinning-tube consists of t\A0 

 parts : a thicker basal ]Dortion, and a thin 

 terminal portion, from the orifice of which 

 the substance of the fibre exudes (PI. 2. fig. 

 10, 10 a, a separate tube). The number of 

 these spinning-tubes varies according to the 

 species, the sex, and the age of the spiders. 

 In some there are more than 1000, in others 

 400, 300, 100, &c., and in others still fewer. 

 The glands which secrete the tenacious tran- 

 sparent secretion are very variable in num- 

 ber, form, and arrangement, and occupy the 

 interstices of the other abdominal viscera, 

 consist of sacs and tubes, lined with nucleated 

 cells, and either simple or variously ramified, 

 terminating in ducts which open at the roots 

 of the spinning-tubes. 



The filaments of which the webs of many 

 spiders are composed are not all alike. The 

 radiating filaments are but little elastic, and 

 are composed simply of one or more threads ; 

 whilst the more numerous filaments con- 

 necting these are covered at tolerably regular 

 intervals with minute spherical masses of 

 glutinous matter (PI, 2. fig. 11), the filaments 

 themselves being highly elastic. These masses 



give the fibres an elegant beaded appearance 

 under low powers of the microscope. The 

 viscid masses cause the more ready adhesion 

 of the filaments to insects which may acci- 

 dentally become entangled in them, and 

 render the sj^ider more sure of holding his 

 prey. 



Propagation. — The Arachnida generally 

 are propagated by sexes, but an exception is 

 formed by the Tardigrada, which are her- 

 maphrodite, and do not possess copulating 

 sexual organs. The sexual apparatus con- 

 sists of two ovarian or seminal sacs, some- 

 times fused together in the middle line ; they 

 are situated in the abdomen, and terminate 

 in two excretory ducts, which usually open 

 at a common orifice placed at the base of the 

 abdomen, or below the cephalothorax. A 

 penis is not generally present ; the seminal 

 fluid is applied to the vulva of the female by 

 the maxillary palpi of the male. 



Spiders are oviparous, and the eggs are 

 enveloped in a cocoon. 



The Ai-achnida may be thus subdivided : 



Section I. Cephalothorax divided into 



four segments ; neither stigmata nor distinct 



respiratory organs present. 



Order 1. Pycnogonida (Polygonopoda). 

 Legs as long as or longer thanthe body ; 

 abdomen rmhmentary (small and conical). 



Genera. — Pycnogonum, Phoxichilus, 

 Phoxichilidium, Pallene, Pariboea, Nym- 

 photi, Endeis, Ammothea. 



Sluggish marine animals, living on the 

 sea-shore under stones or upon marine 

 plants. 



Order 2. Tardigrada (Colopoda). Legs 

 rudimentary, very short, conical, incU- 

 stinctly three-jointed, andwith three orfoiu* 

 claws; abdomen not distinct from the 

 thorax; (aquatic). 



Gen. Emydium, Macrohiotus, Milne- 

 sium. 

 Section 2. Cephalothorax undivided; 



respnatory organs usually distinct, internal 



(but sometimes absent), with two or more 



abdominal stigmata. 



Order 3. Acarina. Abdomen not jointed, 

 fused with the cephalothorax; palpi simple; 

 mouth in many forming a rostrum ; (respi- 

 ratory organs consisting of tracheae or 

 none) . 



Fam.l. Acarea. Head terminated in front 

 by an emarginate labium, or single bifid 

 process ; palpi adnate or adherent to the 

 labium, difficultly distinguished; mandi- 

 bles chelate; no distinct ocelli; feet ge- 



