126 



THE COCKROACH : 



fore modified Leydig's hypothesis. He suggests (Grundziige 

 d. Vergl. Anat.) that the functional rectal folds of Dragon-flies 

 and the non-functional folds of terrestrial Insects are both 

 survivals of tracheal gills, which were the only primitive organs 

 of respiration of Insects. The late appearance of the rectal 

 folds and the much earlier appearance of spiracles is a serious 

 difficulty in the way of this view, as Chun has pointed out. It 

 seems more probable that the respiratory appendages of the 

 rectum of the Dragon-fly larva) are special adaptations to 

 aquatic conditions of a structure which originated in terrestrial 

 Insects, and had primarily nothing to do with respiration. 



Fig. 70. Transverse section of Rectum. X 50. 



The number of the rectal bands (six) is worthy of remark. 

 We find six sets of folds in the gizzard and small intestine of 

 the Cockroach, six bundles of Malpighian tubules, with six 

 intermediate epitheliated bands. There are also six longitudinal 

 bands in the intestine of the Lobster and Crayfish. The 

 tendency to produce a six-banded stomodceum and proctodooum 

 may possibly be related to the six theoretical elements (two 

 tergal, two pleural, two sternal,) traceable in the Arthropod 

 exoskeleton, of which the proctodceum and stomod^eurn are 

 reflected folds. 



