ioo AMERICAN SPIDERS 



Cambrian seas. An alternate theory would have the arachnids de- 

 rived from some land creature, similar perhaps to the sluglike Peri- 

 patus, but of which at present there exists no record. 



The phylogeny of spiders has long been the subject of much 

 speculation, and there is still no general agreement as to the funda- 

 mental paths that were followed. This volume attempts to lay 

 down only the broad features of spider evolution, and acknowledges 

 the inclusion of much speculative matter. 



The phylogeny of any group of animals can be postulated by 

 means of the fossil record, and also by aids from taxonomic classifi- 

 cations, which are frequently indicative of the racial history of a 

 group. 



The ancestral stock from which come all major spider groups 

 originated some time before the Carboniferous Era. These creatures 

 probably bore a close resemblance to the spiders fossilized in the 

 coal measures, with abdomens encased in hardened plates wide ter- 

 gites above, sternites below, and hard, narrow pieces (pleurites) on 

 the sides. Four pairs of similar fingerlike appendages were present 

 beneath the abdomen at about the middle. Just in front of these 

 was a pair of spiracular openings leading to book lungs, and a second 

 similar pair was present farther forward, at the base of the abdomen. 

 Inside the abdomen was an elongate heart into which opened five, 

 six, seven, or even more pairs of ostia, each pair representing a seg- 

 ment and those at the rear much reduced in size. Thereafter, the 

 tendency was to increase the size of the organs in the anterior seg- 

 ments and gradually to suppress the posterior ones, resulting in the 

 loss of some of the ostia and the supporting muscular systems. The 

 gradual reduction and loss of the units of internal segmentation 

 were matched in the external plates, which resulted in an actual 

 migration forward toward the spinnerets of the anal tubercle. 



The cephalothorax of the ancestral spider was relatively long as 

 compared with its width, and was marked by a longitudinal median 

 groove. At the front end were eight eyes set close together on a 

 low tubercle. The legs were of moderate length, quite stout, and 

 each tarsus had at its tip three tarsal claws, the outer paired ones 

 relatively long and smooth, the inner unpaired one short and only 

 slightly curved. The chelicerae were large, set parallel to the long 

 axis of the body, with robust fangs. The gland in the basal segment 

 secreted a weak poison, largely unnecessary since to subdue prey 

 reliance was placed mainly on the strong legs and sharp fangs. 



The earliest spiders were cautious hunters that groped about on 



