SYSTEM OF NATURE. 
101 
two, thus giving a total of four double, or eight single 
classes of apterous articulates. Latreille’s triple division 
of Arachnida is quite unusual, nearly all entomologists 
agreeing in dividing them into pulmonary and trachean. 
The following summary results.— 
FOUR CLASSES. EIGHT CLASSES. 
Crustacea 
Arachnida 
Myriapoda 
Ametabola 
j Malacostraca 
\ Entomostraca 
(Arachnida 
j Acaridea 
(Chilognatlia 
(Chilopoda 
j Thysanura 
\ Anoplura. 
It remains for the skilful anatomist to show that one of 
these groups is separated from its fellows by artificial cha 
racters only. Although I am not sufficiently master of the 
subject to attempt this, I will venture an opinion that the 
Crustacea will eventually be recognised as the single, 
normal, and central group. The contents of this group 
are peculiarly heterogeneous. The Decapoda are, by 
all entomologists, considered the normal division of the 
group. Cyamus and other Laemodipoda approach the 
genera Nymphon and Pycnogonum among the trachean 
Arachnida : several of the minute Branchiopoda — as the 
genus Cyclopsina of Milne Edwards—appear to resemble 
the Thysanura ; and all the lsopoda — perhaps more 
particularly Oniscus armadillo — seem to approach the 
chilognathous Myriapoda, more particularly the genus 
Glomeris. Should these views prove to be correct, Thy¬ 
sanura, Chilognatha and Acaridea will be the subnormal, 
and Anoplura, Chilopoda and Arachnida the abnormal 
