Wo. 1.] EYES OF ARTHROPODS. 163 
sion, he is forced to assume not only that the nerve fibres have 
shifted their attachment from one end of the cell to the other, 
but that new rods have been developed at those ends of the 
cells where, according to all experience, rods are never found. 
His supposition, which implies that inverted rods are not favora- 
ble to perfect vision, which pretends that, in some unexplained 
manner, upright rods get into an unfavorable position, and 
then forces us to-assume a fundamental upheaval of primitive 
relations in order to bring them back to what is practically their 
original condition, is, in my opinion, at present unnecessary. 
Even after making this sacrifice of preconceived ideas, the way 
is no clearer than before, since we have imposed upon ourselves 
the difficult task of explaining why these supposed inverted 
retinal cells are exactly like the upright ones of the lateral 
eye! 
I may be permitted to call attention to one or two facts that 
will perhaps be of service to those who think it necessary to 
make further observations on the eyes of Spiders. My observa- 
tions on the eyes of Astacus, Cymothoa, Vespa, and Acilius 
leave no room to doubt that there are two invaginations con- 
nected with the eyes: one to form the optic vesicle; another, 
the optic ganglion. The presence of two such invaginations in 
types so widely separated suggest that a similar condition prevails 
in Arachnids. If this be so, where is the ganglionic invagina- 
tion? Is it possible that the ganglionic and optic invaginations 
have been confused as in Astacus and Crangon, and the con- 
clusions vitiated thereby? 
The second point I wish to mention is more of a theoretical 
nature, and has to do with the causes of inversion. Professor 
Mark has suggested that the primitive eye was a laterally flat- 
tened cup, with a lens over the slit-like opening, and that either 
the cup was bent to one side, so that the broad surface of one 
wall was folded against the lens, or a second lens had been 
formed, bringing light to the retina from a new direction, and 
this led to the development of the retina from that wall of the 
pit next the new lens. In either case the final result would be 
the formation of a three-layered eye, the middle layer of which 
developed into the retina. A simpler explanation is suggested 
by the condition in eye V. of Acilius, where there are both 
inverted and upright rods; in such an eye it would be a simple 
