THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 31 



The antennae of insects and Crustacea are probably homologous structures and 

 ally the two groups. 



The amnion of insects and arachnids is probably homologous, and allies these two 

 groups ; consequently insects, having characters common to both arachnids and 

 Crustacea, may be placed between arachnids on the one side and Crustacea on 

 the other. It does not follow from this that insects, from the possession of common 

 characters, are the most primitive form, but rather that they are the most hetero- 

 geneous and highly evolved of the arthropod groups. 



The three arthropod groups may not have arisen one from the other ; probably 

 each group arose independently from a common source. 



The tracheae of insects and spiders are probably analogous, not homologous 

 structures. 



This may fairly be concluded from the fact that the tracheae of spiders can be 

 derived from the lung books which it will be remembered are involuted appendages. 

 Consequently we could not expect to find in insects tracheal invaginations occurring 

 in appendage-bearing segments of the body if the tracheae of the two groups were 

 homologous structures. It is needless to say that we do find tracheal invaginations 

 in insects occurring in appendage-bearing segments. The tracheae of insects and 

 spiders are therefore not homologous. 



