THE SUDDEN ORIGIN OF NEW TYPES 429 



tracheae respectively. The anal segment and the indication ot a 

 pre-anal segment would bring up the total number of segments 

 to fourteen, which would closely correspond to the original 

 thirteen or fourteen segments (thorax and abdomen) of insects. 

 Probably the Symphyla stands nearer to the Insectan stock, 

 from which the Tiiysanura branched off, than the other 

 Myriapod classes and in several respects they show primitive 

 characteristics. Possibly they are represented in the fossil state 

 by Scudder's Protosyngnatha, which is constituted by the 

 single Carboniferous form Palceocauipa anthrax^ with ten body- 

 segments, from the Carboniferous of Illinois. 



The essential characteristic of all Myriapods and Insects is 

 their segmental tracheal system, which must have already 

 appeared in their common ancestor ; although at first sight no 

 two structures could seem to differ more widely than tracheae 

 and lung-books (like those of Scorpions and Spiders), yet it 

 seems probable that the former are in reality derived from the 

 latter. It is among the Myriapods that we can even now dimly 

 see how this was brought about, especially by examining the 

 tracheal system of Scutigera ^ (fig. 20), among the Chilopoda, 

 although in their case there is obviously much specialisation. 

 Here we find that each (unpaired and dorsal) stigma opens into 

 an air-sac, from each side of which about 300 radial, branched 

 tracheal tubes arise, closely packed together, forming a kind of 

 lung. This arrangement is essentially similar to the tracheal 

 lungs or book-leaf tracheae of Arachnids, with the sole exception 

 that in these the tracheae are flattened out into lamellae. Since 

 ribbon-like tracheae occur in Araneids, it is evident that book-leaf 

 tracheae could pass without any violent transition into ribbon- 

 like tracheae and finally into the typical tubular tracheae, which 

 are so eminently adapted for a purely terrestrial mode of life. 



The multiple repetition of similar lamellae would again be 

 conducive to inducing an extensive amount of structural 

 variations, capable of leading onwards either to the tubular, 

 branching tracheae of Insects or to the tufted tracheae of 

 PsEUDoscoRPioNiDA and many Araneida. It is obvious that 

 tracheae must have arisen independently in Insects, Arachnids- 



' It is placed in a separate order, the Schizotarsia, by F. G. Sinclair, Ca/nb. 

 Nat. Hist., Myriapods, p. 46, 1895. 



* In Araneids the main trunk of the trachea histologically resembles exactly the 

 general chamber of the lung-sac and is quite different from the trachea of Insects. 



