STRUCTURE OF SPIDERS. 291 



(3-6) Four pairs of walking-legs. The front pair are 

 much used as feelers. The embryo has other 

 four pairs of abdominal legs which abort. 



In front of the anus there are 4-6 spinnerets, from 

 which the silk secretion oozes. 



The nervous system conforms to the usual type, but there 

 is great centralisation, for there is a single ganglionic mass 

 in the thorax from which nerves proceed to the organs. 



There are two or three rows of eyes on the cephalothorax 

 dorsally and anteriorly ; they are like the central eyes of the 

 scorpion. " Auditory hairs " are found on the pedipalps and 

 legs, and a sensory organ, probably olfactory or gustatory, 

 occurs on the basal joint of the pedipalps. It is supposed 

 that some spiders see fairly well ; many trust more to touch 

 and hearing. 



The spider feeds on the juices of insects, and behind the 

 gullet there is a powerfully suctorial region. Five pairs 

 of outgrowths from the mid-gut extend into the bases of 

 the legs. There are also tubular digestive outgrowths, and 

 two excretory Malpighian tubes grow out from the hind-gut. 



Body-cavity, entosternite, and coxal glands generally 

 resemble those of the scorpion. 



The three -chambered heart, with three pairs of valved 

 openings, lies dorsally in the abdominal region, within a peri- 

 cardium in some forms at least. As regards the blood and 

 its circulation there is no great difference between spiders 

 and scorpions. 



Respiration is effected by four chambered tracheae or 

 " pulmonary sacs " in one set (Tetrapneumones, My gale, etc.), 

 by two chambered trachese plus tubular trachese in the 

 great majority (Dipneumones). In every case the openings 

 are abdominal and ventral, but the two chambered tracheae 

 present in most open in the front of the abdomen, the tubular 

 tracheae posteriorly near the spinnerets. 



The males are smaller, and often more brightly coloured 

 than the females. The reproductive organs are usually 

 paired. Two vasa deferentia open beside the apertures 

 of the pulmonary sacs; the oviducts unite and open in 

 the same position. Most females have receptacula seminis 

 independent of, and opening in front of the genital 

 aperture. 



