180 THE HISTORY OF CREATION. 
themselves, however, have in some cases disappeared, and in 
others become transformed into the wings of the Flies. They 
have disappeared entirely in the classes of Spiders and 
Centipedes, and these groups must accordingly be conceived 
of as degenerated or peculiarly developed lateral branches of 
the Fly class, which at an early period branched off from 
the common primary form of Flies ; Spiders probably did so 
at an earlier period than Centipedes. Whether that common 
primary form of all Tracheata, which in my General Mor- 
phology I have named Protracheata, did develop directly out 
of genuine Ringed worms, or at first out of Crustacea of the 
Zoéa form (Zoépoda, p. 177) will probably be settled at some 
future time by a more accurate knowledge and comparison 
of the ontogeny of the Tracheata, Crustacea, and Annelida. 
However, the root of the Tracheata, as well as that of the 
Crustacea, must in any case be looked for in the group of 
Ringed worms. 
The genuine Spiders (Arachnida) are distinguished from 
Flies by the absence of wings, and by four pairs of legs; 
but, as is distinctly seen in the Scorpion-spiders and Taran- 
tulee, they, ike Flies, possess in reality only three pairs of 
genuine legs. The apparent “fourth pair of legs” in spiders 
(the foremost) are in reality a pair of feelers. Among the 
still existing Spiders, there is a small group which is prob- 
ably very closely allied to the common primary form of the 
whole class; this is the order of Scorpion-spiders, or Solifugze, 
(Solpuga, Galeodes), of which several large species live in 
Africa and Asia, and are dreaded on account of their poison- 
ous bite. Their body consists—as we suppose to have been 
the case in the common ancestor of the Tracheata—of a head 
