154 Transactions of the Royal Society of South Africa. 
least variation occurs in Insecta. In Myriapoda and Arachnida it is inter- 
esting to note that the orders showing the 21 somites are just those which, 
either on anatomical or palaeontological grounds, are regarded as being the 
more primitive of their classes. 
There are, then, strong reasons for believing that the Arthropodan is 
primitively and fundamentally constituted by 21 somites. 
In the Arthropoda we find both opisthogoneate and progoneate forms, the 
former including Peripatus, Chilopoda, and Insecta, the latter including 
Crustacea (fundamentally), Diplopoda, Pauropoda, Symphyla, and Arach- 
nida. Such a character must have some significance, and it is of some interest 
that the Myriapoda fall into two divisions in regard to this character — a 
fact supporting a widely accepted idea that this group is not monophyletic 
within itself, especially when it is borne in mind that there is homogeneity 
in this respect in such Arachnida as the Xiphosura and Araneida, which, 
although they are generally placed in the same class, yet differ very much 
from each other in their respective alliances with aquatic and terrestrial 
Arthropoda. 
Farther, it will be found that the genital apertures in the progoneate 
forms of Arthropoda agree wonderfully closely in position, according to 
the scheme of a similar somitic constitution. What slight displacements 
do occur do not by any means exceed those to be found within several 
families of the Oligochaeta, where at the same time they are regarded 
more or less as characteristic of the family. If the positions of the genital 
apertures are to be regarded as being so uniform and of any importance, 
we are forced to one of two conclusions. 
1. That the progoneate Myriapoda and the terrestrial Arachnida have 
evolved such structures as Malphigian tubules and tracheae independent 
of the Opisthogoneate forms (which are all terrestrial and form an easily 
understood group), if the Merostomata are to be grouped with other 
Arachnida in one class. 
2. That all the aquatic and terrestrial descendants of the phylum have 
been evolved from ancestral stock which was aquatic and was characterized 
either in reality or potentially by progoneate and opisthogoneate conditions. 
The opisthogoneate condition has asserted itself in Peripatus, Chilo- 
poda, and Insecta. In this way the classes of the phylum can be made 
monophyletic, and the difficulty re the double evolution of Malphigian 
tubules and tracheae is avoided. 
Such an ancestral stock would, on the lines suggested, be characterized 
by annulate characters approaching those of the Chaetopoda, and would 
consist of 21 somites. 
The occurrence of phleboedesis throughout the phylum Arthropoda is 
so constant that we must regard the presence of a haemacoele of a similar 
nature in the ancestral annulate stock from which the Anthropoda arose. 
