WHAT IS A SPIDER ? 3 



front are one or two pairs of respiratory apertures, and between 

 them the unpaired genital aperture. 



The anus is placed at the extremity of the abdomen on the 

 ventral surface, and is surrounded by two or three pairs of spin- 

 nerets. 



The mouth opens into a short oesophagus with horny walls, 

 whichg terminate in a dilated radiating, suctorial stomach, from 

 which are given off four or five pairs of caeca running into the 

 legs. The intestine is narrow, and opens into a short dilated 

 rectum, which receives a pair of much branched urinary or Mal- 

 pighian^canals. Salivary glands open into the anterior portion of 

 the oesophagus. The liver is very large and much branched, 

 opening into the intestines. 



The vascular system is well developed. The blood is colour- 

 less. The heart is a chambered dorsal vessel, situated in the 

 abdomen, from which an aorta runs forward into the cephalothorax 

 giving off lateral arteries to the legs, jaws, brain, and eyes. In 

 the fore part of the cephalothorax, these arteries reunite, surround- 

 ing the brain, and forming the abdominal aorta, which runs back- 

 wards into the abdomen. The blood, after making its way 

 through the tissues and bathing the lung-sacs, re-enters the heart 

 by three pairs of lateral valves. 



Respiration is effected partly by lung-sacs, composed of a 

 number of delicate lamellae and partly by tracheae or air-tubes, 

 There are one or two pairs of lung-sacs situated in the anterior 

 portion of the abdomen, and opening by slit-like stigmata. The 

 tracheae open by a pair of stigmata, further down, sometimes 

 quite at the extremity of the abdomen. 



The nervous system is concentrated in a cerebral ganglion, or 

 brain, and a large ganglionic mass, situated in the thorax and sup- 

 plying nerves to the legs and abdomen. 



Spiders possess an apparatus for the production of a viscid 

 fluid, which has the property of hardening into silk on exposure to 

 the air. This apparatus consists of numerous glands pouring 

 their secretions through fine pores on to the surface of the spin- 

 nerets, which are from two to four pairs of conical papillae placed 

 behind the anus. The apex of these spinnerets is surrounded by 

 stiff bristles and hairs, and is dotted with numerous horny tubes, 



