122 The American Naturalist. [February, 
which will be referred to again below.. I would now present 
the following scheme: 
Phylum Arthropoda. 
Sub-Phylum I, Branchiata. 
Class I, Crustacea. 
Sub-Class I, Trilobite. 
Sub-Class, II, Eucrustacea. 
Class II, Acerata. 
Sub-Class I, Gigantostraca. 
Sub-Class II, Arachnida. 
Sub-Phylum II, Insecta. 
Class I, Chilopoda. 
_ Class II, Hexapoda. 
Sub-Phylum III, Diplopoda. 
Incertæ Sedes— 
Pycnogonida. 
 Linguatulina. 
Pauropoda. 
Tardigrada. 
Malacopoda. 
The various papers by Lankester, McLeod, Laurie and my- 
self have, I think, clearly shown that the older grouping of the 
Arthropoda into Branchiates and Tracheates is not justified 
by the facts of structure and ontogeny; that tracheæ are not 
homologous structures in all Arthropods which possess them, 
and that the old group of Tracheata is polyphyletic in orig!” 
Since classification must represent the various lines of descent, 
the old must therefore go. There remain many points which 
must be investigated anew, but I feel confident that further 
research will support, in its main features, the classification 
adopted above, and considered more in extenso below. 
PHYLUM ARTHROPODA. 
I am not prepared to discuss the validity of this grouP: 
although for reasons that will appear below, I am inclined to 
believe the great divisions which I recognize are but remotely 
related to one another, and it may yet be proved, as I pi 
gested several years ago (’83), and as von Kennel believes; tha 
