CHAPTER VIII 

 AKTHKOPODA 



Classification adopted 



Trilobita. Pantopoda. 



Prototracheata (Onychophora). Myriapoda. 



Crustacea. Insecta. 



Arachnida. Tardigrada. 



THE great group of the Arthropoda comprizes at least two-thirds of 

 the species of animals now existing, besides a large proportion of 

 those extinct forms whose remains are preserved as fossils. Of the 

 existing species belonging to the group the greater number are 

 included in the enormous classes of Crustacea, Arachnida, and Insecta. 

 The Myriapoda is a class which can only be described as a lumber- 

 room, since it contains some species which have close affinities 

 with the Insecta and others of widely different relationship. The 

 Prototracheata, Pantopoda, and Tardigrada are small classes including 

 comparatively few species. The Trilobita is a large class, but all the 

 species comprizing it are now extinct and are known only from their 

 fossil remains. 



It is generally conceded by zoologists that an Arthropod is 

 merely a further development of the annelidan typo of structure. 

 The two groups Arthropoda and Annelida agree in having their 

 bodies built up by a metameric repetition of similar segments, and in 

 the structure of the central nervous system. The most primitive of 

 living Arthropoda are the Prototracheata or Onychophora ; they 

 possess a series of so-called nephridia metarnerically arranged which 

 correspond to the coelomiducts of Annelida, and they are on this 

 account excluded from the Arthropoda altogether and reckoned as 

 Annelida by some naturalists. 



The true relationship which Prototracheata, and through them 

 the rest of the Arthropoda, sustain to the Annelida was made clear 

 by the classic researches of Sedgwick on the development of 

 Peripatus capensis (1885-1888). We cannot select this or any other 

 species of Peripatus as a type for special description because of the 

 extreme rarity of the animals belonging to this genus, but it is 



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