PEOPLING OF LAND AND SEA 169 



The lungs of Thelyphonidae, Phrynus, and Spiders, differ in 

 no way from those of the Scorpions, and MacLeod's L 

 explanation consequently extends to them also. Lamy, 

 moreover, has followed step by step in the case of the Spiders, 

 their metamorphosis into tracheal tubes. 2 This metamorphosis- 

 is complete for the second pair of lungs of the Dysderidae and 

 Segestriinae, which are normal Spiders in all other respects. 

 Two tracheae co-exist with the lungs in all the other Spiders, but 

 are carried back towards the posterior extremity of the body, 

 and there is only a single median orifice placed in front of the 

 spinnerets. In the Galeodidae, Field Spiders and Pseudo- 

 Scorpions, the metamorphosis affects the whole pulmonary 

 apparatus, hence these Arachnida are known as trachean. 

 Mites or Acarina, generally small in size and often parasites, 

 likewise breathe through their tracheae, and thus seem to be 

 degenerate Arachnida. However, the position and the number 

 of the respiratory orifices which vary according to the genus 

 and which may disappear altogether, render the assimilation 

 of their tracheae to the respiratory organs of other Arachnida 

 rather uncertain so far as present knowledge goes. That does 

 not affect the fact that the Arachnida present a special mode of 

 forming their internal respiratory organs, different from that 

 met with in Peripatus, and that they represent a second group of 

 land immigrants, likewise archaic, and dating back to the 

 Silurian period. Scorpions, as a fact, have been found in Silurian 

 deposits, particularly in the island of Gothland. The Arachnida, 

 moreover, belong to a class of Arthropods in which the first 

 appendages of the body, anterior to or near the mouth, are 

 still at least partially utilized for functions other than the 

 retention or mastication of food, and which with creatures 

 like Pterygotus, Eurypterus, Limulus, and the Trilobites, con- 

 stitute the sub-class Merostomata. 



With centipedes or Myriapods we come to a class manifestly 

 derived from the true Crustaceans, in which the first five pairs 

 of appendages are specialized for tactile or masticatory 

 functions. Here, however, the segments carrying these 

 appendages, more or less distinct in the Crustaceans — are 

 combined in a single mass, whose limits we are unable to 

 distinguish, and which we call the head. All the other segments 



1 LIX. 2 LX, 836. 



