Mesosoma 



eg- 



776 CLASS V. ARACHNIDA. 



9. Genital operculum 



10. Pectines or 1st gill-book 



11. 1st lung-book or 2nd gill-book 



12. 2nd 3rd 



13. 3rd 4th 



14. 4th 5th 



15. 5th mesosomatic segment with no appendages 



16. 1st metasomatic ,, ,, ,, 



17. 2nd ,, ,, ,, ,, ,, 



18. 3rd ,, ,, ,, ,, ,, yMetasoma 



19. 4th 



20. 5th 



It is interesting to note that there are also 20 pre-anal 

 segments in the higher Crustacea such as Astacus, and the same 

 number is thought by some to characterize the Insecta. In 

 the more specialized Arachnids, however, a reduction in the 

 number of segments usually takes place. The anus opens behind 

 the last metasomatic segment, and behind it in the higher 

 forms is the spine in Limulus and the poison-sting of the 

 Scorpion. 



In the primitive aquatic Arachnids respiration is performed 

 by certain modified appendages which take the form of gills 

 (v. Fig. 512), each provided with a number of leaf -like laminae 

 between which the water circulates. These gill-books are re- 

 placed in the more primitive terrestrial forms by lung-books 

 which do not project into the air but are sunk in pits in the 

 ventral surface of the body. These pits open to the exterior 

 by a pair of stigmata on each segment and the air enters here 

 and circulates between the laminae of the lung-book. 



Their development shows that these lung-books are primi- 

 tively limbs. In still higher forms the lung-books are replaced 

 by tracheae, though both lung-books and tracheae may coexist, 

 as in certain spiders. The tracheae of Arachnida are believed 

 by some authors to have been derived from lung-books. They 

 differ from those of Insecta and Myriapoda and resemble those 

 of Peripatus in being tuft-like and in not anastomosing 

 (except in Solifugae, p. 83). On these and other grounds it 

 has been maintained that the Arachnidan tracheae are not 

 homologous with those of Insecta, but that they have 



