94 THE APODID^E PARTI 



exceptions independent groupings of the sensory cells 

 under different forms of cuticular irregularities. 



Returning to the eye of Apus, it is of special 

 interest to find that the eyes of all the Crustacea 

 which we assume to have descended from Apus may 

 be referred back to different groupings and modifica- 

 tions of the ommatidia, whose first development in 

 Apus we have endeavoured to describe. 



The formation of the corneal facets above the 

 crystal cones may be due to a further utilisation 

 of irregularities in the thickness of the cuticle which 

 remains after the separation of the crystal cones. In 

 this way, we think, the gradual development of the 

 Crustacean eye may have gone hand in hand with 

 the thickening of the cuticle to form the exoskeleton 

 characteristic of the class. We have two highly 

 plastic elements, the hypodermis, with its scattered 

 sensory and pigment cells, and the thickening cuticle. 

 We cannot help thinking that it was the latter which, 

 coming between the sensory cells and the source of 

 stimulation, took the lead in the formation of the 

 different kinds of Arthropodan eye. 



Before dismissing the subject of the development 

 of the Crustacean eye we feel that some apology is 

 necessary for treating it so shortly and so lightly. 

 We have not attempted to work through the enor- 

 mous literature on the structure and development of 

 the Crustacean eye. 1 Our object here has been to 



1 While these pages have been passing through the press we have 

 had occasion to read Watase's admirable paper on the " Morphology 

 of the Compound Eyes of Arthropods." It was especially interesting 



