THE COMPOUND EYE. 517 



ture, the rhabdome (PL XXXVI., Fig. i, rli), usually sur- 

 rounded by six sheathing cells, enclosed in a pigmented cellular 

 sheath. The sheathing cells I regard as the generating cells 

 of the rhabdome. The cone (cc) is also surrounded by the 

 continuation of the pigmented sheath, which forms a kind of 

 iris (i) around its apex. Each ommateum may therefore be 

 regarded as a transparent tube, surrounded by pigment cells. 



The rhabdomes are sometimes solid rods, but are more fre- 

 quently cuticular sacs, filled with fluid. 



Tracheae. Between the ommatea in Insects there are usually, 

 perhaps always, numerous tracheae, which have their long axes 

 parallel with the long axes of the ommatea. These tracheae 

 are regularly arranged, so that each ommateum may be said to 

 be surrounded by a circle of tracheal vessels. Beside the 

 tracheae, the ommatea are separated from each other by narrow 

 blood-sinuses. 



The Retina (PL XXXVI., Fig. i, rt). The term ' retina' has 

 been applied by me to a layer of nervous elements and pigment 

 cells which lies internally to the basilar membrane, and which 

 is connected with the optic ganglion by the decussating fibres, 

 already described as the optic nerve. 



As this layer consists of rod-like elements, identical with 

 those of the simple eye, in which the nerve-fibres obviously 

 end, and upon which a dioptric picture can be shown to fall, 

 this term is not only appropriate but necessary, unless it is to 

 be entirely discarded in relation to the invertebrate eye. 



It is now ten years since I first published my views on the 

 compound eye, and I was then sanguine enough to believe 

 that the evidence on which they rest was strong enough to 

 ensure their immediate acceptance. This has not, however, 

 been the case, although my opinions are surely and steadily 

 gaining ground in this country, and are readily accepted by all 

 those who are competent to judge, and who have taken the 

 trouble to examine some of the numerous sections which I 

 possess, and which show the structure of my retina as distinctly 

 as the drawings which are given in this work. 



Received Views. The view usually held as to the structure 



