A.EACHNIDA. 485 



In the Spiders (Araneida) the abdomen is swollen and is joined 

 to the cephalo-thorax by a short stalk. In the Scorpionidce, on the 

 contrary, the long abdomen is joined to the cephalo-thorax by its 

 whole breadth, and is divided into a broad segmented prse-abdomen 

 and a narrow, very movable post-abdomen, which is also seg- 

 mented. In the Mites LAcarinci) the abdomen is unsegmented and 

 fused with the cephalo-thorax. In the Pentastomida the entire 

 body is elongated, ringed and vermiform, with four (two pairs of) 

 hooks in place of the appendages; these animals are known as 

 Linguatulida, and might be placed, on account of their parasitism, 

 amongst the intestinal worms. 



The marked reduction of the cephalic region, which is without 

 true antenna? and possesses only two pair of oral appendages, is 

 characteristic of the Arachnida. The anterior pair of cephalic 

 appendages (chelicerse), which are used as jaws, have been regarded 

 as modified antennas ; but it is perhaps more natural to regard them 

 as morphologically equivalent to the mandibles of Crustaceans and 

 Insects. These anterior jaws or chelicerpe are either chelate, in 

 which case the claw-like terminal joint can be moved against a 

 process of the preceding joint (Scorpions, many Acarina), or sub- 

 chelate, when the last joint is folded down upon the next like the 

 blade of a pocket-knife upon the handle (Spiders). 



The cheliceraa may also have the form of stylets, which are 

 enclosed in a tube formed by the second pair of jaws (Mites). The 

 latter, which constitute the second pair of appendages of the head or 

 the pedipalpi, consist of a stout basal joint and a palp, which has fre- 

 quently the form and segmentation of a leg. This either ends with 

 or without a claw or with a chela (Scorpions). In the true Spiders 

 there is an unpaired plate, the lower lip, between the basal joints of 

 the two pedipalpi and belonging to the same segment as the latter. 

 The four following pairs of appendages of the thorax are ambulatory 

 legs. The first of them is sometimes modified in form and elongated 

 like a palp ; its basal joint may function as a jaw. The legs consist 

 of six or seven joints, which, in the higher forms, have been called by 

 the same names as the analogous regions of the Insect leg. 



The internal organization of the Arachnida shows hardly fewer 

 differences than does that of the Crustacea. The nervous system 

 may have the form of a common ganglionic mass around the oeso- 

 phagus (Mites), and may even possess only a simple commissure 

 above the oesophagus (Pentastomida). As a rule, however, there is a 

 distinct separation between brain and ventral cord, the latter showing 



