THE ARTHROPODS OR INSECTS. 1 73 



to form the main-class of the trachea-breathing Arthropoda, 

 or Tracheate Insects (Tracheata). 



In all animals with articulated feet, as the name indicates 

 the legs are distinctly articulated, and by this, as well as by 

 the strong differentiation of the separate parts of the body, 

 or metamera, they are sharply distinguished from Einged 

 worms, with which Bar and Cuvier classed them. They 

 are, however, in every respect so like the Ringed worms 

 that they can scarcely be considered altogether distinct 

 from them. They, like the Ringed worms, possess a very 

 characteristic form of the central nervous system, the so- 

 called ventral marrow, which commences in a gullet-ring 

 encircling the mouth. From other facts also, it is evident 

 that the Arthropoda developed at a late period out of 

 articulated worms. Probably either the Wheel Animalcules 

 or the Rino-ed worms are their nearest blood relations in 



O 



the Worm tribe. (Gen. Morph. ii. Plate V. pp. 85-102.) 



Now, although the derivation of the Arthropoda from 

 ringed Worms may be considered as certain, still it cannot 

 with equal assurance be maintained that the whole tribe of 

 the former has arisen out of one branch of the latter. For 

 several reasons seem to support the supposition that the 

 Gilled Arthropods have developed out of a branch of articu- 

 lated worms, different from that which gave rise to the 

 Tracheate Arthropods. But on the whole it remains more 

 probable that both main-classes have arisen out of one and 

 the same group of Worms. In this case the Tracheate Insects 

 — Spiders, Flies, and Centipedes — must have branched off at 

 a later period from the gill-breathing Insects, or Crustacea. 



The pedigree of the Arthropoda can on the whole be 

 clearly made out from the palaeontology, comparative ana- 



