hear'infj on the orvjin of Insects. IGl 



glands ; however this may be, the remarkable difference 

 in the position of the genital openings exhibited by the 

 different groups and, very generally, by the opposite sexes 

 of Arthropoda is intelligible on the hypothesis that all the 

 members of the sub-kingdom have descended from some 

 worm-like creature, provided in every somite of its body 

 with a pair of segmental organs or nephridia, and that 

 different pairs of these organs have, in different descend- 

 ants of this hypothetical ancestor, been converted into the 

 genital aperture and ducts. 



6§. The gonapophyses of female BlattidcB are homo- 

 logous, part for part, Avith the appendages of the eighth 

 and ninth abdominal somites in the female of Machilis. 

 No one has, so far as I know, ever suggested that the 

 exarticulate setose styles (fig. 4) movably attached to the 

 hinder extremity of the ninth abdominal sternum in the 

 males of most Blattidce are homologous with the abdo- 

 minal appendages of the Thysanura, and yet the resem- 

 blance between the two is very striking ; nor have any 

 representatives of them yet been discovered in the fe- 

 males. Some months ago, while dissecting a species of 

 Bluttidce, I detected at the extremities of the outer 

 branches of the posterior bifid pair of appendages beneath 

 the skin that was about to be cast, a small bud, with the 

 skin that had shrunk away from it drawn out into a 

 shrivelled and curled filament, and I dissected specimen 

 after specimen of the same species, until I found the fully- 

 evolved appendages shown in fig. 9, a. The append- 

 ages are reduced to mere rudiments in the eighth somite. 

 (Fig. 7, «.) For further information see the rather full 

 explanations of the accompanying woodcuts. 



M 2 



