48 PRIMITIVE ANIMALS 



which case they must be looked upon as highly 

 metamorphosed gills that have been retracted into 

 the body, and have nothing to do with the true 

 Insect tracheae in derivation. Certain Spiders (the 

 Dipneumonous Spiders), besides the respiratory sacs 

 which are probably homologous with the lung-books 

 of Scorpions, possess medially situated tracheae 

 which have had an entirely separate origin, namely 

 from the sinking below the skin of certain tendons 

 originally intended for the attachment of muscles. 

 The Arachnid respiratory organs are therefore not 

 only entirely independent of the Insect tracheae, but 

 have themselves been derived from more than one 

 source. 



We have seen, then, within the Arthropod phylum 

 that the two great terrestrial groups of the Insects 

 and Arachnids stand apart from one another, and 

 that the more ancient and primitive forms of Arach- 

 nids, so far from affording a link with Peripatus and 

 Insects, are marine animals showing no resemblance 

 to these latter groups. 



The same may be said of the class Crustacea. 

 They too occupy an isolated position in the phylum, 

 and we know of no transitional forms between the 

 most ancient Crustacea and Peripatus, Insects or 

 Arachnids. Here again, as in the case of the 

 Arachnids, we have an abundantly represented, well- 

 preserved but very ancient fossil group, the Trilobites 



