AND MORPHOLOGY OF APHIS. 231 



ence to these views, is the common Cockroach {Blatta orientalia), an insect which I 

 can recommend as admii'ably 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 aU these facts, the conehision 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 

 confii-m 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 sterntun 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 boundaiy 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 careftd 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 Avhich belong to the two posterior pairs of ambulatory limbs are clearly visible 

 on the posterior pai-t of the cephalothorax, and these limbs are strongly distingmshed 

 from the anterior two pairs by the absence of the triangular processes dii-ected 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 maxillae of any other Articulate animal. 



Another exceedingly natural demarcation between the two anterior and two posterior 

 pairs of ambulatory limbs is afforded by the so-called ' diaphragm ' which divides 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 legs are attached to distinct 

 segments. 



There is no necessity to enter into any disquisition upon the homology of the append- 



