374 The Pedigree of Insects 



pods are probably comparable to the false feet or 

 parapodia of marine worms, and that tracheal air- 

 tubes may have originated from skin-glands. The 

 genital ducts of arthropods are almost certainly to 

 be regarded as modified kidney -tubes (nephridia) 

 which have been retained in various segments in 

 the different classes (2l6, 217)- It may be doubted, 

 however, if the most highly organised of living 

 worms should be regarded as representing the an- 

 cestors of Arthropods. And if the suggestions of 

 this chapter as to the relationships of Insects, 

 Crustaceans, and Arachnids be justified, the im- 

 mediate ancestors of the Arthropods cannot have 

 been, as commonly believed (2l6, 217), " richly 

 segmented" animals. 



Origin of Insects. — Our comparison of the insect- 

 orders with one another, and of the Class with other 

 classes of arthropods, enables us to trace to some 

 extent the history of the group. We look back to 

 a time far earlier than the oldest of our fossil-bearing 

 rocks and imagine the primitive arthropods — marine 

 animals with twenty segments, whereof nineteen 

 carried paired limbs. Already the first two pairs 

 were specialised as feelers, the third as mandibles, 

 the fourth as maxillse. From this ancient stock the 

 ancestors of the Crustacea branched off, retaining 

 both pairs of feelers j the number of segments varied 

 immensely in the Entomostraca, but remained con- 

 stant, the form and function of the limbs becoming 

 highly modified, among the Malacostraca. Another 

 group, losing both pairs of feelers, and taking for 

 the most part to life on land, became the primitive 

 Arachnids. A third group, also taking to a terrestrial 

 life, but losing only the hind pair of feelers, were 

 the progenitors of Centipedes and Insects. If the 

 Millipedes also sprang from this stock they must 



