CHAPTER IX 



CLASS V. ARACHNIDA* 



This is a much more diverse Class t than the Insecta, and con- 

 tains animals which vary within as wide limits as do those 

 included in the class Crustacea. Many of the Orders are small, 

 but the Spiders and Mites comprise a very large number of 

 species, and in some species a very large number of individuals. 



There are no antennae. The chelicerae are the first pair of 

 appendages and the pedipalpi the second. The mouth lies 

 between the bases of the pedipalps. In the more typical Orders 

 a varying number of appendages starting from the third or 

 the pedipalpi have at their proximal ends processes, called 

 gnathobases, which project inward round the mouth and help 

 to hold and break up the food. 



In some divisions of the Arachnida the number of segments is 

 definite as is the case with the Malacostraca. These segments are 

 grouped into tagmata or categories of a higher order, and there are 



* Pickard Cambridge, Encyclopaedia Britannica, ix Ed., 1872, Article 

 Arachnida. E. R. Lankester* ibid., x Ed., and The Structure and Classi- 

 fication of the Arachnida, Quart. J. Micr. Sc., xlviii, 1905, p. 165. Hensen. 

 Organs and Characters in Different Orders of Arachnida, Entomol. Meddel, 

 iv, p. 137. G. Newport, Nervous and Circulatory Systems in Myriapoda 

 and Macrurous Arachnids, Phil. Trans., 1843. Claus, Degeneration in 

 the Acari and Classification of Arthropoda, Anz. Ak. Wiss., 1855, and 

 Ann. Nat. Hist (5), xvii. 1886, p. 364, and xix, p. 225. Blanchard, Les 

 Arachnides, in L' Organization du regne animal. Gaubert, Recherches 

 sur les Arachnides, Ann. Sci. Nat. (7). xiii. 1892. C. Koch, Die Arachniden, 

 16 vols., Nuremberg, 1831-48. Koch, Keyserling and Sorensen, 

 Die Arachniden Australians, Nuremberg, 1871-90. R. I. Pocock, Arach- 

 nida of British India and " On African Arachnida," Proc. Zool. Soc. 

 and Ann. Nat. Hist., 1867-1900. Simon, Les Arachnides de la France, 

 7 vols., Paris, 1874-81. Hist. Nat. des Araignees, 2 vols., Paris, 1892-6. 

 Thorell, Arachnida from the Oriental Region, Ann. Mus. Genova, 1877-99. 



t We differ from Lankester in regarding the Trilobites as more nearly 

 allied to the Crustacea, especially to the Phyllopoda than to the Arach- 

 nids. As we do not include them here our group Arachnida is equivalent 

 to his group the Nomeristica. E. R. Lankester, Quart. J. Micr. Sci., 



xlviii, 1905, p. 165. 



774 



