ARACHNIDA. 



[ G6 ] 



ARACHNIDA. 



Char. Head, body, and oreater part of tlae 

 abdomen covered by a sbield-like carajjace, 

 "which is deeply notched behind ; eyes two, 

 sessile and approximate ; a single pair of 

 minute, short, styliform and two-jointed 

 antennas ; legs, sixty pairs, the first pair 

 furnished with three long jointed bi'auclies, 

 extending beyond the carapace, the rest 

 branchial ; body composed of numerous 

 rings ; two long jointed caudal appendages. 



A. cancriformis. Freshwater, in stagnant 

 pools ; brownish yellow; length 2i inches. 



A. produdiis. Not British ; an elongated 

 oval lamina between the two caudal appen- 

 dages. 



BiBL. Baird, B)-if. Eijtom. p. 18 ; R. Lan- 

 kester, Qu. Mia: Jn. 1881, xxi. p. 343. 



ARACHNI'DA. — A class of animals con- 

 taining the spiders, scorpions, Szc. 



Char. Head united with the thorax, 

 fonuing a cephalothorax ; antennfe none ; 

 eyes simple (ocelli) ; legs eight, jointed. 



The integument of tlieArachnida is usually 

 soft and leathery, rarely horny or brittle, and 

 consists principally of chitine. Two layers 

 may usually be distinguished, an outermost 

 or cuticle, which is tlie firmest and strongest, 

 and not unfrpqueutly exhibits a cellular ap- 

 pearance in the extremities and the cepha- 

 lothorax. The cuticle of the abdomen of 

 the Aranene, Acarina, kc. presents very 

 beautiful wa-vv' or undulating lines, some- 

 times surrounding the roots of the hairs &c. 

 concentrically, and arising, in some cases at 

 least, from the existence of folds (PI. 6. figs. 4 

 & 5). The cuticle of the Arachnida is fre- 

 quently covered with warty and bulbous 

 excrescences, bristles and simple or feathery 

 hairs, and sometimes with scales. 



The innermost cutaneous layer consists of 

 a very delicate and almost colourless mem- 

 brane, of a finely granular or fibrous appear- 

 ance, close beneath which is situated a 

 layer of pigment granules and cells, which 

 are visible through the general integument, 

 and to which the beautiful colours of many 

 of the Arachnida are owing. 



The organs surrounding the mouth varv 

 in structure in the difierent f\imilies. In the 

 Spiders, two mandibles are situated at the 

 front of the head. These consist of two 

 joints — a basal, very thick one (PL 6. fig. Ga 

 & 7 re), and a terminal, curved and shaiply 

 pointed one (fig. 6 & 7 h). The latter is 

 traversed by a canal terminating at its apex, 

 through wliich the secretion of a poison- 

 gland passes into any body transfixed by the 

 claw. These mandibles are perhaps, strictly, 



modified antennfe . Next come two maxillary 

 palpi (fig. 7 1), which do not differ in struc- 

 ture from the legs, except in their tarsi 

 being composed of a single joint, generally 

 terminated in the females by a small hook, 

 but in the males of more complicated struc- 

 ture : the basal joints of these palpi are 

 enlarged and project forward, forming the 

 maxillae (fig. id); these are stated to con- 

 tain certain glands, opening on the inner 

 side. of the upper face. In the scorpions, 

 the mandibles and maxillary palpi terminate 

 in pincers or forceps. Lastly a labium, 

 situated between the niaxillse (fig. 7 e), and 

 consisting of a single piece. 



The mouth in the other families is de- 

 scribed under the respective heads. 



The eyes are simple (ocelli, .stemmata), 

 but they are absent in the parasitic Acarina ; 

 they consist of a simple arched cornea, a 

 spherical lens and a concavo-convex vitreous 

 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 con- 

 striction ; but sometimes the head, thorax, 

 and abdomen are fused together. 



The legs of the Arachnida, which are 

 attached to the cephalothorax, do not co- 

 incide 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 last two joints belong 

 to the tarsus, the tibia then consists of two 

 joints, of which, in some (the scorpion and 

 Phrifnus) the first, in others the second, is 

 the longest. The preceding long joint is the 

 femur, next to which comes 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, cor- 

 responds to the coxa of insects. The last 

 joint of the tarsus usually supports three 

 curved hooks or claws (PI. G. fig. 8), which 

 are frequently toothed on the concave mar- 

 gin, and in some a membranous vesicular 

 or hairy cushion (pulviilus) on its under 

 side. The most characteristic feature of the 

 Arachnida consists in the division of the 

 tibia into two unequal pieces. 



The first pair of legs is supposed to repre- 

 sent the labial palpi of insects. 



The alinienfari/ canal is mostly short and 

 straight. In the Arauere the oesophagus 

 enlarges into a prismatic muscular expan- 

 sion just before its termination in the sto- 

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