AND MORPHOLOGY OF APHIS. 231 
ence to these views, is the common Cockroach (Blatta orientalis), 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 appa- 
ratus 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 col- 
leterial 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 
Insecta, 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 seven- 
teen post-oral somites besides the sting (which is plainly the homologue of the telson in 
the Crustacea) in this Arachnidan. 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 seg- 
ments 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 cepha- 
lothorax, 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 terga which belong to the two posterior pairs of ambulatory limbs are clearly visible 
on the posterior part of the cephalothorax, and these limbs are strongly distinguished 
from the anterior two pairs by the absence of the triangular processes directed towards 
the mouth, which characterize the bases of the latter. Indeed, the anterior ambulatory 
legs, by means of these processes, take part in the formation of the oral cavity as com- 
pletely as do the maxillæ of any other Articulate animal. ae 
Another exceedingly natural demarcation between the two anterior and two posterior 
pairs of ambulatory limbs is afforded by the so-called ‘diaphragm’ which pees the 
thoracic from the cephalic cavity, and whose attachment corresponds with the interval 
between these two sets of appendages. 
In Galeodes, the two posterior pairs of ambulatory 
Segments. 
There is no necessity to enter into any disquisition upon the homolo 
legs are attached to distinct 
gy of the append- 
