and Homologies of some Articulates. 327 



and this thorax is the abdomen of Arachnology. It is seg- 

 mented in some modern species, while in others the subdi- 

 visions have become obsolete or are but faintly indicated. The 

 abdomen of the Eurypterid however exists as a slender jointed 

 thread in Geralinura of Scudder, of the Carboniferous, which 

 has its Illinois, and also Bohemian, species and has survived 

 till now, in the modern Telyphonus. 



Derivation of Myriapods and Insects. — Myriapods although 

 inferior to Insects, are as yet known only from the early De- 

 vonian. The Devonian species, and also those of the Car- 

 boniferous, are of the Millepede or lower doubly-Vnultiplicate 

 section of Myriapods with one exception, that of the remark- 

 able few-jointed, caterpillar-like Palceocampa of Meek and 

 Worthen. 



The fact of a line of succession from Worms to Myriapods 

 and from Myriapods to Insects has not been proved by geo- 

 logical discovery. The derivation of Myriapods from some 

 type of Annelids is zoologically suggested, as long since recog- 

 nized, by the apparently transitional form of Perijiatus, a low- 

 grade Myriapod resembling much the larve of some Insects, 

 and by the like multiplicate structure of Annelids and Myria- 

 pods. It might be inferred also from the resemblance of the 

 Palseocampa of the Illinois Carboniferous to the caterpillar of 

 an Insect of the genus Arctia, as remarked by Scudder. 



Myriapods are regarded as the precursors of Insects on 

 account of their approximate resemblance to the latter in 

 antennae and the appendages of the mouth, and because also of 

 the wormlike form of most Insect larves, these larves appear- 

 ing to be survivals of the Myriapod stage. In the change 

 from an Annelid and Myriapod to an Insect, the mitltiplicate 

 feature disappeared and the number of parts became essentially 

 the fixed normal number of the type, both as regards the body- 

 segments and their jointed appendages. 



The rise of grade from the Myriapod to the Insect involved 

 the appropriation of the three body-segments of the Myriapod 

 bearing the three anterior pairs of feet (which correspond 

 normally to half the body segments of the head of an Isopod 

 Crustacean) for forming the isolated middle section of the body 

 called the thorax, and the suppression of all the other pairs of 

 feet. In both Spiders and Insects, the change involved also a 

 general concentration of the structure toward the cephalic 

 nervous center, that is a shortening of the range of cephalic 

 control, and especially the distance to the posterior limit of 

 locomotive action. Compared with a Crab, the highest type 

 in the Crustacean series, its superior, an Ant, is a very little 

 thing. 



