332 ARACHNOIDEA AND PAL&OSTRACA. 



the wanderers who spin little, and the sedentary forms who 

 spin much. The body consists of an unsegmented cephalo- 

 thorax and a soft unsegmented abdomen, separated by a 

 narrow waist. The chitinous cuticle varies in hardness, 

 hairiness, and colouring ; it has as usual to be moulted as 

 the spider grows. Thus the young garden spider moults 

 eight times in its first year. 



There are six pairs of appendages : 



(i.) The two jointed chelicerae or falces, whose terminal joint bends 

 down on the other in " sub-chelate " fashion, and is perforated by the 

 duct of a poison gland. 



(2.) The leg-like, usually six-jointed, non-chelate pedipalps, whose 

 basal joint helps in mastication, while the terminal joint in the male 

 expands as a reservoir for the spermatozoa and serves as a copulatory 

 organ. 



(3-6.) Four pairs of terminally clawed walking legs. The most 

 anterior pair are much used as feelers. In the embryo there are four 

 pairs of abdominal appendages which abort. 



The nervous system is of the usual Arthropod type, but 

 shows much centralisation. Thus the ventral ganglia are 

 fused into one large centre in the cephalothorax, a condition 

 comparable to that in crabs. There are two or three rows 

 of simple eyes on the cephalothorax, whose focal distance is 

 very short, spiders trusting most to their exquisite sense of 

 touch by which they discriminate the various vibrations on 

 a web line. The senses of smell, hearing, and taste are 

 also present, but little is known in regard to the organs. 



Body cavity, endosternite, and coxal glands generally 

 resemble those of scorpions. 



The spider usually sucks the blood and juices of its prey, 

 and behind the gullet lies a powerfully suctorial region, 

 strengthened by chitinous plates, and worked by muscles. 

 From the small mid gut arise five pairs of long caeca, a pair 

 running forwards and a pair passing into the bases of each 

 pair of legs and then back again. These caeca sometimes 

 anastomose. Further back, the mid gut gives off numerous 

 digestive outgrowths which fill a large part of the abdomen. 

 Their secretion digests proteids. Terminally there is a 

 large cloaca, and where the intestine joins this, four ex- 

 cretory Malpighian tubes are given off. 



A three-chambered heart, containing colourless blood, 

 lies within a pericardium near the dorsal surface of the 



