14 



THE BRITISH NATURALIST. 



[January 



AIR-BREATHING ARTHROPODS. 



BY LINNAEUS GREENING. 



If we adopt the modern democratic principle of attaching great 

 importance to numbers, the Arthropods are unquestionably the most 

 important of the great groups into which animals are dividec^ Practi- 

 cally, however, no one carries out to their logical conclusions these 

 democratic principles, but even after making due allowance for that pride 

 of race which causes us to regard our noble selves as the apex of the 

 animal kingdom, it still remains true that students of natural history, who 

 are particularly strong in entomology, should take a special interest in 

 studying the relationship of the different classes of Arthropods. These 

 are usually regarded as six in number — viz., Onychophora, Myriapoda, 

 Thysanura, Arachnida, Insecta, and Crustaceans. 



.A much more fundamental, and presumably natural, division of the 

 arthropod kingdom is into the two sub-kingdoms of gill-breathers and 

 air-breathers, i.e., Branchiata and Tracheata. The former is co-extensive 

 with the class Crustacea, the latter includes the five other classes with 

 which we are to deal in the following pages. 



The majority of writers on the relationship of the Arthropods insist on 

 comparing the members of the five air-breathing classes with the 

 Crustaceans, and this tendency has certainly led to the enunciation as 

 scientific truths of a considerable number of undemonstrable and probably 

 erroneous statements. 



If it b^ true that all the air-breathing Arthropods are much more 

 closely related to one another than any of them are to any of the gill- 

 breathers, the study of the comparative anatomy of air-breathers and 

 crustaceans can have merely an academic interest, and can throw little, 

 if any, light on the real affinities of these animals. It is in the highest 

 degree probable that all the widely different members of the four higher 

 classes of air-breathing Arthropods are the lineal descendants of 

 terrestrial forms, superficially resembling our modern centipedes, and 

 structurally intermediate between them and the Onychophora, which are 

 the slightly modified descendants of the remote ancestors of all the 

 Tracheata. 



Before going further it will be well to define what is meant by an j 

 Arthropod. The literal meaning of the word is joint-foot, and the term 

 happily expresses the tendency, so marked throughout the group, for 

 segmentation to occur not only in the body but in all the appendages ; I 

 even so delicate an organ as the antenna of a butterfly is as clearly a 

 segmented structure as is the pincer of the scorpion. In many Arthropods, 

 presumably in all, chitin enters into the composition of the exo-skeleton, 

 which throughout the group is the hardest part of the body. 



