Palaeozoic Arachnida of North America. 85 



ORDER ARANEAE 



Head completely fused with thorax. Abdomen with pedicel, seg- 

 mented in the lowest forms, in all higher forms unsegmented. Che- 

 licera retroverte. Pedipalpi pediform, six-jointed, their coxa in 

 higher forms with a maxillarly lobe. Legs seven-jointed, movably 

 articulated to a sternum. Patella present in all legs and palpi. 

 Trochanters all one-jointed. Spinning glands in the abdomen with 

 numerous ducts opening on the spinnerets ; spinnerets always on the 

 abdomen, either in the middle of its ventral surface as in Liphistiidae, 

 or at the end, immediately in front of the anus. Respiratory organs 

 in form of two pairs of lungs in lower forms, or one pair of lungs and 

 a pair of tracheal tubes, in higher forms ; sometimes without lungs, 

 but with two pairs of tracheal tubes, or with one pair of lungs, but 

 without tracheal tubes. Spiracles in all cases on the ventral surface 

 of the abdomen. Genital opening in both sexes on the ventral sur- 

 face of the abdomen, not far from its anterior end. Sperm receptacles 

 in the female either connected with the uterus, or blind, often with 

 a complicated chitinous structure or epigynum. Organs of copulation 

 in the male on the pedipalpi, the terminal joint of which is specially 

 modified for this purpose. Usually eight eyes (sometimes six, four, 

 two or none) in two or three rows on the cephalothorax. Morphologi- 

 cally they represent two groups : one pair of median eyes with direct 

 retina and two groups of lateral eyes with inverted retina. All spiders 

 are oviparous. They eat insects and other spiders. Their instincts 

 are highly developed and manifold. Some have burrowing habits, 

 others make snares, some are true vagabonds. W'ith very few ex- 

 ceptions they lead terrestrial life, and those living under water or 

 between the tides are not really aquatic, since they use air for 

 breathing. 



Key to the Sub-Orders of Araneae. 



1. Abdomen segmented. Four pairs of spinnerets in the middle of 

 the abdomen on its ventral surface 



Mesothelae 

 + abdomen not segmented. Spinnerets immediately in front of 

 the anus 2 



2. Chelicera so articulated, that the fangs move parallel to the plane 

 of symmetry of the animal. Two pairs of lungs 



Mygabmorphae 

 + Chelicera so articulated, that the fangs move in a plane clearly 

 intersecting the plane of symmetry. One pair of lungs and one 



