ARACHNIDA. 



[ 56 



ARACHNIDA. 



body, with a cup-shaped retina, and a layer 

 of pigment corresponding to the choroid. 



The cephalothorax is usually separated from 

 the abdomen by a well-marked constriction. 



The legs of the Arachnida do not coincide 

 exactly with those of insects. They usually 

 consist of seven segments tapering towards 

 the end, so that the tarsi are less distinct 

 from the other parts than in insects. If we 

 suppose that the two last joints belong to the 

 tarsus, the tibia then consists of two joints, 

 of which, in some (the scorpion and Phrynus] 

 the first, in others the second is the longest. 

 The preceding long joint is the femur, to 

 which comes next an annular or inverted 

 conical joint, corresponding to the trochanter 

 of the six-footed insects. The first, broad, 

 usually inversely conical joint, which is 

 adherent to the cephalothorax, corresponds 

 to the coxa of insects. The last joint of the 

 tarsus usually supports three curved hooks 

 or claws (PI. 2. fig. 8), which are frequently 

 toothed on the concave margin, and in some, 

 a membranous vesicular or hairy cushion 

 (pulvillus) on its under side. The most cha- 

 racteristic feature of the Arachnida consists 

 in the division of the tibia into two unequal 

 pieces. 



The alimentary canal is mostly short and 

 straight. In the Araneae the oesophagus 

 enlarges into a prismatic muscular expansion 

 just before its termination in the stomach; 

 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 Araneae 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 caeca, 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 Aranese 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 lungs and tracheae together. 



The tracheae of the Acarina are remarkably 

 delicate, so that the spiral fibre is with great 

 difficulty distinguishable. They arise usually 

 in an unramified 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. 



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 (PI. 2. fig. 9). When examined by 

 reflected light, they reflect 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 no 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 



