24 ANA TOM \ ' AXD MORPHOLOG V OF INSECTS. 



their subsequent union form the perfect worm, the remainder 

 of the epiblast being detached and shed. Moreover, similar 

 phenomena occur amongst echinoderms, a fact recognised dis- 

 tinctly by Dr. Weismann, who compared the metamorphoses 

 of the Diptera with those of the Echinodermata, which at that 

 time were regarded as a form of alternate generation (^meta- 

 genesis), a view strongl}', and I think successfully, combated 

 by Barrois.* 



Nevertheless the change from the larva to the nymph in the 

 flies is so complete that not one single organ is common to 

 the larva and the imago. In the words of Harvey,-|- there is a 

 complete return to the egg and a re-development of the insect. 

 Taken as an isolated phenomenon, the development of the 

 nymph in these insects might be regarded as an alternation of 

 generation ; but viewed in the light of comparative morphology, 

 it appears only as an extreme case of metamorphosis, con- 

 nected, by a long series of transitional phenomena, with simple 

 ecdysis, accompanied by a gradual change of external form 

 and internal structure. 



* Barrois, ' L'Embryologie des Nemertes,' Ann. Sci. Nat., sen iii., 

 torn, vi., 1S77. 



t Harvey, Gulielmi, opera omnia a Coll. Med. Lond., edita 1766, ' D .- 

 Generatione Animalium,' ex. ii., p. 183. 



