400 Causes and Course of Organic Evolution 



forms — after attaining at times gigantic size — perished soon 

 after the close of the palaeozoic epoch. The fresh-water ento- 

 mostracan Crustacea and the land or air-breathing x\rachnida 

 persisted and have branched out into numerous phyletic rami- 

 fications. 



So, on reviewing the living arachnidan groups, it can be 

 said that, while one to three genera of king-crabs are recognized 

 and are marine, at least forty-six genera of Scorpionida, twenty- 

 two of Pedipalpi, six hundred and fifty -one genera of Araneida 

 or spiders, one hundred and seventy genera of Solipugse, 

 Chelifers, Acarids, and related orders, and nine genera of 

 Tardigrada or Pentastomida are land or rarely fresh-water. 

 The Pycnogonida alone, that include about 42 genera, have 

 branched off into a marine life. Thus, out of 886 arachnid 

 genera, at most 45 can be regarded as marine, and these seem 

 evidently to have originated from fresh- water or land ancestors. 



The Centipedes and Millipedes, that together formed the 

 older group of the Myriapoda, include 67 genera, all of which 

 are land animals and air-breathers, though some live at times 

 by the shore. Fossil forms have been traced back to the 

 devonian age, but with no suggestion even then of a marine 

 origin. All existing evidence favors the view that they, along 

 with the Insecta, were probably derived from an annelidan 

 ancestry, the genus Peripatus being one that connects the 

 present group with the Annelida. But weighty reasons might 

 be advanced for regarding both as derivatives from the ento- 

 mostracan line, and so still more primitively from Rot if era. 



The enormous group of the Insecta, that includes at least 

 350,000 living species, alike in its entire structure, history, 

 and present distribution is so typically terrestrial that only 

 vague suggestions have been made as to its possible derivation 

 from some marine ancestry. The order Aptera is now gen- 

 erally regarded as the most ancient and primitive, while of 

 it and of the orthopterid order fossil remains have been found 

 in devonian, silurian, and even in ordo\dcian rocks, while 

 PalcBoblattina has been regarded as a primitive type of the 

 cockroaches. In common with the millipedes, the arachnids. 



