PALEOZOIC TIME — CARBONIC. 723 



thoracic. Air-breathing was another new feature ; and for this purpose parts 

 of the body had air-vessels or tracheae which opened by breathing holes, or 

 spiracles, on the under side of four of these " thoracic " segments. In the 

 later true Spiders the body had lost its Eurypteroid abdomen, but had still, 

 in Paleozoic species, its distinctly segmented thorax ; and this thorax is the 

 abdomen of Arachnology. (It is segmented in some modern species, while 

 in others the subdivisions have become obsolete or are but faintly indicated.) 

 The abdomen of the Eurypterid, however, exists as a slender jointed thread in 

 Oeralinura of Scudder, of the Carboniferous, which has its Illinois and also 

 Bohemian species, and has survived till now in the modern Thelyphonus. 



10. Derivation of Myriapods and Insects. — Myriapods, although inferior 

 to Insects, are as yet known only from the early Devonian. The Devonian 

 species, and also those of the Carboniferous, are of the Milleped, or lower, 

 doubly-multiplicate section of Myriapods, with one exception, that of the 

 remarkable few-jointed, caterpillar-like Pa.lmocampa of Meek and Worth en. 



The fact of a line of succession from Worms to Myriapods and from 

 Myriapods to Insects has not been proved by geological discovery. The 

 derivation of Myriapods from some type of Annelids is zoologically suggested, 

 as long since recognized, by the apparently transitional form of Peripatus, a 

 low-grade Myriapod resembling much the larve of some Insects, and by the 

 like multiplicate structure of Annelids and Myriapods. It might be inferred 

 also from the resemblance of the Palaeocampa 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 tlie latter in antennae and the appendages of the 

 mouth, and because also of the worm-like form of most Insect larves, these 

 larves appearing to be survivals of the Myriapod stage. In the change from 

 an Annelid and Myriapod to an Insect, the multiplicate 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 appropria- 

 tion of the three body segments of the Myriapod bearing the three anterior 

 pairs of feet (which correspond normally to half the bod}"^ 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 concentra- 

 tion 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 

 limits of locomotive action. 



While in the Cockroach, and related low-grade species, there is no proper 

 metamorphosis, in higher Insects, as they rise in grade, the larval stage is 

 lower and lower in embryonic level, becoming, in the highest, destitute of 

 locomotive organs ; and this fact suggests that the larval stage results from 

 an attendant retrograde embryonic change toward, and to, a line parallel with 



