64 SPIDERS: THEIR STRUCTURE AND HABITS. 



which respiration is cutaneous — />., by the general surface of the 

 body, — or else by trachea, which are air-tubes, opening on the 

 surface of the body by stigmata, or spiracles, and branching freely 

 as they penetrate the interior. The eyes in this division never 

 exceed four. (2) Piibnonaria^ in which respiration is by pulmo- 

 nary sacs alone, or by these and tracheae conjointly, and the eyes 

 are generally six or eight in number. 



To the former division belong the Sea-Spiders or Fodosomata, 

 Mites or Acari, and Phalatigidoe or Harvest-Spiders, distinguished 

 by the length of their legs. 



To the latter belong the higher Arac/mida, as Scorpions and 

 Spiders. The Scorpions are possessed of a segmented abdomen 

 terminating in a hooked claw, perforated at its point by the duct 

 of a poison-gland which lies at its base. There is no line of 

 demarcation between the abdomen and cephalothorax, and they 

 have strong nipping-claws, or chelae. 



The AfaneidcB, or true Spiders, (called also Dimerosomata^ 

 from their bodies showing two distinct divisions), are characterized 

 by the union of head and thorax into one mass, which is 

 named the cephalothorax, and by a soft unsegmented abdomen 

 attached to the former by a peduncle. They breathe by pul- 

 monary sacs in combination with tracheae. The head bears 2, 4, 

 6, or 8 simple eyes; they have no chelate limbs, and their 

 abdomen terminates with a spinning-apparatus instead of a sting. 

 These are the principal points of difference between Scorpions 

 and Spiders. 



In treating of these latter, it will be best to commence with 

 the internal structure, as it is by this, rather than by outward form, 

 that the divisions of the animal kingdom are ruled. 



Spiders possess a system of circulation and respiration dis- 

 tinguishing them from insects, and giving them a higher rank in 

 the scale of creation. The blood is colourless, and like that of 

 fishes holds in suspension oval corpuscles. The heart is a long 

 muscular vessel, placed lengthwise in the upper part of the 

 abdomen, enclosed in a pericardium, and having four chambers. 

 An artery runs through the peduncle, separating in the cephalo- 

 thorax into three pairs : of which the upper pair sends off vessels 

 to the eyes and mouth, the middle pair to the digestive organs, 

 and the third to the legs. These arteries re-unite in the forepart 

 of the cephalothorax, and form one canal, which runs backwards 

 along the lower part of it, and of the abdomen, to the spinning- 

 organs, sending out small branches on its way (Plate 7, Fig. i). 

 The blood is then passed on through channels analogous to veins 

 into receptacles communicating with the breathing-organs, where 

 it is oxygenised, and so returns to the heart. 



The pulmonary sacs, or gills, two in number, are involutions 



