THE APPENDICULATE PHYLUM 45 



with the exception of the eyes, it is an Arthropod, 

 and approaches most nearly the Centipede or 

 Myriapod type of organization. 



Although we are thus able in some measure to 

 link up the Annelids and Arthropods together, the 

 various classes of Arthropods stand in a rather 

 isolated position with regard to one another. We 

 may derive the Myriapods from a Peripatus-like 

 ancestor, and the immense class of Insecta probably 

 comes from a similar source. Like Peripatus these 

 animals are all typically air-breathing forms, living on 

 dry land and taking in the air by means of openings 

 on the surface of the body (stigmata) which lead into 

 tubes or tracheae distributed to all parts of the body. 

 The other two great classes of Arthropods, the Arach- 

 nida and Crustacea, stand in a doubtful relation to 

 the typical Tracheates mentioned above and to one 

 another. The modern and most highly developed 

 Arachnida, viz. the Spiders, are true land-animals 

 and they resemble Insects in possessing tracheae, 

 but it seems certain that the Spiders' tracheae are 

 an independent acquisition and not a legacy from 

 a common ancestor of the Insects. For we know of 

 certain very remarkable Arachnida, of a much more 

 primitive general structure than Spiders, which in- 

 habited the ancient seas and breathed by gills very 

 much in the manner of Crustacea, and these animals 

 have left a descendant, a link with the past, which 



