1894.] eet Classification of the Arthropoda. 127 
Arachnids into exact correspondence. In the case of the 
Crustacea the corresponding line passes behind the third 
maxilliped of the Decapod. . 
If it should, however, be shown (as many believe) that the 
Crustacean metastoma has its own somite, the line will be 
thrown forward to behind the second maxilliped, and it will 
correspond to the line of division between the head and thorax 
of the Edriophthalmia. 
Since the older ideas of numerical sequence are better known, 
I have used them in the following discussion rather than that 
based upon the neuromeres. Thus in the Hexapods somite 
(or appendage) I=Neuromere II; in the Arachnid and Xipho- 
sures somite I = Neuromere IV; in the Crustacea somite I = 
Neuromere III. 
The morphology of some other organs call for a moments’ 
consideration. Prominent among these are the vasa Malpighii. 
These are usually regarded as characteristic of the “Tracheates,” 
and their presence in the Arachnids has been adduced as a 
strong argument for their association with the Hexapods. It 
has been, however, pretty conclusively shown that these 
organs are not homologous throughout the Arthropod phylum, 
for in the Hexapod they are derived from the hind-gut, and 
are therefore ectodermal, while in the Arachnida, as Loman 
(’86-7) has shown, they are derivatives of the mesenteron and 
are consequently entodermal. Their similarities are those of 
homoplassy rather than of homology, and the only argument 
that can be drawn from the occurrence in these forms is that 
Arachnids and Hexapods are not closely related. Similar 
organs with similar functions have been described in various 
Edriophthalmia, but we are yet in doubt as to their origin. 
The studies of Spencer (’85) represent them as without chitin- 
ous intima in the Amphipods. They may, therefore, be ento- 
dermal. A detailed study of the region of the hind-gut of 
certain Decapods might give results interesting in this con- 
nection. 
The trachee furnished another instance of homoplassy. 
These organs furnish the chief ground for the group called 
“Tracheates,” since in most they form the sole means of res- 
