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SYDNEY J. HICKSON. 
that each palisade-cell has a cylindrical axis, which he says 
may be such a chitinous tube. 
I have examined my sections of Musca, both transverse and 
longitudinal, very carefully, but can find no evidence of the 
existence of any such tube, and I have also, by a process of 
maceration described below, satisfied myself that no chitinous 
structures of this kind are present in the optic tract behind. 
The tubes of Sarcophaga may possibly be tracheal tubes 
similar to those I have described in the peri-opticon of Eris- 
talis, and the appearance which Carriere describes (but does 
not figure) in Musca may be due to the curious effects which 
the pigment often pi’oduces ( v . figs. 8 and 9). 
The recent paper by Lowne upon the retina of insects I 
have referred to above and criticised elsewhere (8). My 
description of the optic tract of insects is quite different from 
his, as is my opinion of the function of the various parts. 
§ 7. Some general Considerations upon the Eye and 
Optic Tract of Insects. 
The layer composed of the retinulse and rhabdoms cannot, as 
Ciaccio (4), Berger (2), and others have shown, be considered 
to be the equivalent of the retina of other animals. It is only 
part of the retina, namely, that part of it which bears the 
nerve-end cells, and corresponds, functionally at any rate, to 
the layer of rods and cones of the Vertebrate eye. In Verte- 
brates and in those Invertebrates with large, highly-organised 
eyes, such as Cephalopods and such genera as Pecten and 
Spondylus among Lamellibranchs, and Alciope among the 
Polychgeta, the retina contains, in addition to the rods and 
cones, several layers of nerve-cells, ganglion-cells, neurospon- 
gium, &c., which are very different in order and arrangement 
for different groups of animals, but very constant in the 
individual genera and species. 
In Arthropoda we also find behind the basilar membrane of 
the ommateum similar complicated nervous strata, which in 
all probability perform the same function. 
