THE ARTHROPODS OR INSECTS. 173 
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 Ringed 
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 Ringed worms are their nearest blood relations in 
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 paleontology, comparative ana- 
