THE COMPOUND EYE. 517 
ture, the rhabdome (Pl. XXXVL., Fig. 1, 7h), 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 (¢) aroynd 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. 
Trachesw.—Between the ommatea in Insects there are usually, 
perhaps always, numerous trachee, which have their long axes 
parallel with the long axes of the ommatea. These trachez 
are regularly arranged, so that each ommateum may be said to 
be surrounded by a circle of tracheal vessels. Beside the 
trachez, the ommatea are separated from each other by narrow 
blood-sinuses. 
The Retina (Pl. XXXVI., Fig. 1, 7t)—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 
