368 SYNOPTIC COLLECTION. 



of mouth parts are indicated at this stage by larger buds 

 (figs, i, 2). The latter continue to grow, while the ab- 

 dominal feet are soon lost. 



Comparatively speaking, the order of Arachnida does 

 not exhibit those marked variations of structure which 

 are seen in most orders. While, however, the general 

 features remain essentially the same, the details vary 

 according to the habits of the different families. One of 

 the oldest fossil spiders, Arthrolycosa antiqua Harger (PI. 

 908), while possessing the form and the appendages of 

 most Arachnida. yet apparently has certain other struc- 

 tural features which ally it with the more generalized spi- 

 ders of the present day, such as Eurypelma ( = Mygale) 

 (No. 909, 9 ; No. 910, $). 



The position and character of the mandibles in this 

 ancient spider (PI. 908) seem to indicate that these 

 organs moved vertically like the mandibles of the hairy 

 Eurypelma. The four pairs of walking-feet are also 

 similar to those of Eurypelma, and altogether it is most 

 probable that both genera belong to the same suborder, 

 the Tetrapneumones, which has four air sacs like the 

 scorpions and only four spinnerets, two of which are 

 long and bent up behind the abdomen. These organs 

 are not so useful in these hairy spiders as in some of the 

 more specialized Arachnida, since they do not spin elab- 

 orate webs for catching their prey, but live in nests made 

 in the ground. One genus (Cteniza) of this group makes 

 the ingenious trap-door nest (No. 911). These nests are 

 usually constructed in the moist earth which afterwards 

 hardens ; they are lined with soft silk from the spinner- 

 ets, and the door is skillfully hinged, fitting snugly into 

 the opening. By means of the claws on the feet or the 

 fangs of the mandibles 1 the trap-door (see No. 911) is 

 raised and the spider backs down the tube, the door clos- 



1 McCook says "apparently the fangs." Amer. Spiders and their 

 Spinning-work, III, 1893, p. 30. 



