ARACHNID A. 



[ 67 ] 



ARACHNIDA. 



mach; the stomach splits just behind the 

 above apparatus into two branches, which 

 curve forwards and form a ring, from which 

 five pairs of diverticula pass to the roots of 

 the legs and palpi. 



Salivary glands are present, consisting in 

 the Aranese of a transparent glandular mass 

 situated in a cavity above the palate ; also a 

 hepatic apparatus, in the form of a compact 

 mass, consisting of a number of ramified and 

 closely-crowded casca, containing the hepatic 

 cells and opening at about the middle of the 

 alimentary canal in four short ducts. This 

 hepatic apparatus was formerly mistaken for 

 the fat-body. In the Tardigrada, Acarina, 

 and some others the liver is represented by 

 the granule-cells, usually brownish yellow, 

 of the walls of the diverticula of the stomach. 



The poison-glands of the Araneae consist 

 of two long, sometimes slightly curved blind 

 sacs, the walls of which are surrounded by 

 a simple spiral layer of muscular fibres. 



Circulatory System. In the lower Arach- 

 nida, as the Tardigrada, Acarina, &c., there 

 is neither dorsal vessel nor blood-vessels. 

 Hence in these there is no regular circulation 

 of blood, but the nutritive fluid or the blood 

 is distributed free in the interstices of the 

 body, and is irregularly moved backwards 

 and forwards, propelled in the cavity of the 

 body, and into the extremities, by muscular 

 movements and the contractions of the in- 

 testinal canal. 



In the Araneae there is a dorsal vessel 

 consisting of a spindle-shaped tube lying 

 principally in the abdomen, constricted at 

 intervals and furnished with lateral apertures 

 and valves. This heart sends off lateral and 

 terminal arterial branches, which gradually 

 become lost. There are no veins ; but the 

 further course of the blood takes the form 

 of lacunal currents, which re-enter the heart 

 at the valvular orifices. 



In the scorpions, there are veins as well 

 as arteries. 



Respiratory System, In the Tardigrada 

 and some parasitic Arachnida, Demodex, 

 Sarcoptes, Acarus, &c., no tracheae or other 

 respiratory organs have yet been discovered ; 

 hence the respiration must be cutaneous. The 

 higher Arachnida breathe either by tracheae 

 (many Acarina), or by lungs, or by both. 



The tracheae of the Acarina are remarkably 

 delicate, so that the spiral fibre is with great 

 difficulty distinguishable. They arise usually 

 in an uuramified bundle from two stigmata, 

 which are sometimes situated anteriorly, be- 

 tween the front legs, as in the Hydrachnea, 



and much concealed, at others at the sides 

 of the body above the third pair of feet, as 

 in the Gamaseae, or behind the last pair, as 

 in the Ixodeae. They are usually more 

 tufted than branched as in insects. 



In the Hydrachnea, which live in the wa- 

 ter, and do not rise to the surface to respire, 

 the tracheae must possess the power of ab- 

 sorbing the air from the water. In the 

 Araneae, the lungs consist of rounded sacs 

 situated at the anterior part of the under 

 surface of the abdomen, and open externally 

 by a transverse slit. At the outer convex 

 surface of each lung-sac there are a number 

 of thin but firm triangular or rhomboidal 

 plates, like the leaves of a book, closed to- 

 gether (PL 6. fig. 9). When examined by 

 reflected light, they present a silvery lustre ; 

 whilst by transmitted light they appear dark 

 violet or almost black. Each of these plates 

 consists of a fold of the skin, between which 

 the air of the sac is widely distributed : they 

 contain on blood-vessels; hence probably the 

 blood brought by the arteries is poured out 

 around the lungs, and so bathes the lung- 

 plates. The position of the lung-sacs is in- 

 dicated externally by a triangular and horny 

 cutaneous plate, at the posterior margin of 

 which the respiratory fissure exists. Behind 

 these fissures there are two other openings, 

 the orifices of a tracheary system which 

 does not differ materially from that of such 

 as have tracheae only. 



Nervous System. Varies in degree of 

 complexity. In its simplest form it exists 

 as a single oesophageal ganglion, sending off 

 radiating branches ; and in its most compound 

 forms it presents a large cephalothoracic 

 bilobed ganglion, and one or two ventral 

 ganglial chains or cords. 



The primitive nervous fibres and gan- 

 glion-cells are very small and delicate. 



Spinning-organs. These organs, by means 

 of which the Araneida 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 somewhat flattened at the summit; and 

 usually the middle pair consist 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- 



