236 



DR WALTER H. GASKELL. 



Strictly speaking, of course, the visual cells with their elon- 

 gated processes have uo right to be called neurons : I only use 

 Parker's phraseology in order to show how closely the two 

 retinas agree even to the formation of synapses between the fine 

 drawn-out processes of the visual cells and the neurons of the 

 ganglion of the retina. 



Again, the retina of the crustacean possesses a membrane 

 remarkably like the external limiting membrane of the verte- 

 brate eye, so that the layer of rods in the simpler forms of 



— -Cornea 



Cone cells 



---■—- Retinular cell 



habdome 



ment cell 



:''0>iL li"""^i ^jJ tiasemeut membrane 

 ^fif^\://k /^J\= ext. limiting membrane 



J nu-clei of retin\ilar cells 

 "\ = L-xf. nuclear layer 



^J\-Hh.- terminal tibte layer 

 Fig. 3. — Oramatidium of Gammarus (after Parker). 



Crustacea such as Gammarus, etc., is separated from the nuclear 

 layer of the visual end cells by this membrane, as is shown in 

 the accompanying figure taken from a paper by Parker.^ 

 Berger- describes the same membrane in Branchipus, and 

 considers it to be cuticular, and therefore belonging originally 

 to the cuticular covering of the same layer of hypodermal cells 

 as gave origin to the cuticular rods of the retinal cells, 



1 Parker, " The proto-mechanical changes in the retinal pigment of Gammarua," 

 Bull, of the Harvard Mus. of Comp. ZooL, vol xxxv., 1899, p. 143. 

 '■* Op. cit. 



