TYPE ARACHNID A. 



449 



is unseginented, as is also the abdomen, which is an oval, 

 spherical, or sometimes irregularly-shaped region which 

 narrows suddenly anteriorly so as to be much narrower than 

 the cephalothorax. The chelicene project somewhat in front 

 of the cephalothorax and each consists of a broad basal joint 

 and a terminal strong claw which may be flexed upon the 

 basal joint, and has opening at its tip the duct of a poison- 

 gland (Fig. 206, pg) which lies in the cephalothorax. The 



ht 



oc 



ce 



sd 



rb 



spg 



tr 



FIG. 206. DIAGRAM OF STRUCTURE OF A SPIDER (after LET-CHART). 



ao = aorta. 



ce = cerebral gauglkm. 

 ch = chelicera. 

 dg = digestive gland. 

 gp genital pore. 

 Jit = heart. 

 Ib = lung-book. 

 mt = Mulpigbian tubule. 

 oc = eye. 

 ov = ovary. 



pe = pedipalp. 

 pg = poison-gland. 



rb = rectal bladder. 



rs = receptacuhim semiuis. 

 s stomach. 



sd = stomach diverticulum. 



sp = spinneret. 

 spg spinning-glands. 



tg = thoracic ganglion. 



tr = trachea. 



pedipalps of the females are leglike structures usually with a 

 terminal unguis, but in the male are more or less swollen to 

 serve as accessory organs in copulation. The four pairs of 

 seven-jointed legs are all similar in structure and serve for 

 walking, differing in relative length in different genera. In 

 the embryo the abdomen is distinctly segmented and bears 

 five or six pairs of rudimentary appendages, the more ante- 

 rior of which later disappear, while the two or three posterior 

 pairs persist as the spinnerets (sp), so called from the occur- 

 rence on them of the openings of the ducts of the spinning- 

 glands (spg}. 



These are very numerous and open at the apices of the 

 spinnerets, each gland producing a fluid secretion which 

 quickl} r hardens on exposure to the air to form a silken 



