TYPE ARAOHNIDA. 



449 



is unsegmented, 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 chelicerse 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 



it m 



Pre, 



spg 



306. — Diagram of Stkuctueb of a Spider (after Letickart). 



ao = aorta. 



ce = cei-ebral ganglion. 

 ch = chelicera. 

 dg = digestive gland. 

 gp = genital pore. 

 hi = heart. 

 lb = lung-book. 

 mi = Malpigbian tubule. 

 oc = eye. 

 ov = ovary. 



pe = peilipalp. 

 pg = poison-gland. 

 rb = rectal bladder. 

 rs = receptacuhim semiuis. 

 s = stomach. 

 sd = stomach diverticulum. 

 sp = spinneret. 

 spg = spinning-glands. 

 ig = 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 

 quickly hardens on exposure to the air to form a silken 



