74 ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS 
future investigators will bring only a confirmation of their general 
accuracy. 
The only adult Insect, besides Apfhzs, which I have studied with 
sufficient care in reference to these views, is the common Cockroach 
(Blatta ortentalts), an insect which I can recommend as admirably 
adapted for investigation. Here it is very easy to find the eleven 
abdominal somites, and to satisfy oneself that the vulva is placed 
between the eighth and ninth, and that the two outer elongated 
pieces of the curious clasping apparatus for the ovisacs are formed 
by a modification of parts of the ninth somite. The smaller and 
inner processes, on the other hand, are clearly developed from the 
sternum of the tenth somite, while the lateral anal valves represent 
the eleventh somite. 
I have found that while the vulva opens between the eighth and 
ninth somites, the aperture of the spermatheca is situated on the 
sternum of the ninth, and that of the colleterial glands on the sternum 
of the tenth somite. 
In the male the complex penis is formed by a modification of the 
tenth somite, and the aperture of the vas deferens is on the sternum 
of this somite, or between it and the eleventh. 
Weighing all these facts, the conclusion to which they point seems 
obvious, viz. that in /ysecta, as in Crustacea, the typical number of the 
somites is twenty. 
I have shown above that the development of the Scorpion proves 
that there are seventeen post-oral somites besides the sting (which is. 
plainly the homologue of the telson in the Crustacea) in this Arach- 
nidan. If we make the same assumption for the Scorpion as for the 
Insect, that one of the antennary somites is abortive, we shall have a 
total of twenty somites here also. The anatomy of the adult Scorpion 
appears to me fully to confirm this view. Beginning at the hinder 
end, we find, including the telson, six segments behind those which 
carry the respiratory apertures. Of these there are four; and in the 
three posterior, the sternum has nearly the same length as the tergum ; 
but in the anterior one the sternum is much longer than the tergum. 
Furthermore, these sterna at first seem to occupy the whole space up 
to the posterior boundary of the cephalothorax, while, on the dorsal 
side, two narrow terga lie between the tergum corresponding with the 
anterior sternum and the cephalothorax. 
It appears, therefore, as if there were two more terga than sterna 
in the abdomen ; but on more careful investigation, the missing sterna 
show themselves as the supports of the pectines and of the genital 
aperture in front of these last curious organs. Indications of the 
