3l6 AN INTRODUCTION TO THE STUDY OF FOSSILS 



a consideration of relationships, see page 309. 5. Pedipalpida 

 (Pennsylvanian to present). 6. Palpigrada (living). 7. Kus- 

 tarachnida (Pennsylvanian). 8. Solpugida (Pennsylvanian to 

 present). 9. Ricinulei (Pennsylvanian to present). 10. Pseu- 

 doscorpionida (Oligocene to present). 11. Araneida (Penn- 

 sylvanian to present). Cephalothorax and abdomen are 

 distinct but neither is segmented. At the posterior end 

 of the abdomen are tubes (spinnerets), the tips of 

 each perforated by very many openings from which comes 

 a fluid which hardens upon contact with the air; the union 

 of these hundreds of microscopic threads forms the ordinary 

 thread just visible to the naked eye. This order includes 

 the true spiders. 12. Anthracomarti (Pennsylvanian). Prob- 

 ably ancestral to the Pedipalpida and Phalangida. 13. Hap- 

 topoda (Pennsylvanian). 14. Phalangiotarbi (Pennsylvanian). 

 15. Phalangida (Pennsylvanian to present). A well-known 

 example is daddy longlegs (Phalangium) ; this hves upon small 

 insects. 16. Acarina (Oligocene to present). Degenerate 

 arachnids. These include the mites and ticks. 



1. Distinguish the Arachnida from the Insecta. 



2. What difference between the w^ater-breathing and air- 

 breathing divisions of the Arachnida ? 



3. Distinguish between book-gills, book-lungs and tracheae. 

 Name animals of which these are characteristic. 



4. Compare the branchiate Limulus with the tracheate 

 Scorpio. In what respects do they (a) agree, (b) differ ? 



5. In what respects do fossil scorpions from the Silurian unite 

 still more closely these branchiate and tracheate portions ? 



6. What fossil forms agree with Scorpio more closely in ab- 

 dominal segments ? 



7. Why are the eurypterids and the limulids (Order Xipho- 

 sura) supposed to have already separated in pre-Cambrian time ? 



8. Are the trilobites related to the limulids, eurypterids 

 or scorpions ? Reasons. 



9. How early in geologic time are the Scorpionida known ? 

 the Araneida ? 



10. How is a spider's thread formed ? 



