THE EYE OF PERIPATUS 169 
rise to a completely separated vesicle, part of which becomes 
a subcorneal layer. On the other hand the retinal layer can 
be traced into the ectoderm. 
With this marked difference we must also note that the lens 
in the Insecta and the Myriapoda is directly continuous with the 
cuticle and is indeed a local thickening of the same, whilst in 
Peripatus it is secreted within the vesicle. 
The modern work confirms, therefore, the statements of 
Lankester (5), when in his article on the structure and 
classification of the Arthropoda he adds, *... the Chaetopod 
eye, which is found only in the Onychophora where the true 
Arthropod eye is absent. ‘The essential difference between 
these two kinds of eye appears to be that the Chaetopod eye 
(in its higher developments) is a vesicle enclosing the lens, 
whereas the Arthropod eye is a pit or series of pits into which 
the heavy chitinous cuticle dips and enlarges knobwise as 
a lens’. 
Thus whilst we can homologize the cuticle, cornea, sub- 
corneal layer, &c., of Peripatus with parts of the simple eyes 
of the Myriapoda and Insecta, the Peripatus eye is not primitive 
so far as the dioptrical parts are concerned, but has developed 
along its own lines and resembles that of the highly-developed 
Chaetopoda. The Eye of Peripatus has, however, not evolved 
very far, and its retina is quite simple and indeed not at all 
unlike that of the median ocelli of Helophilus (one of the 
Diptera) or of the eye of Scolopendra. In both these examples 
we have retinas consisting solely of visual cells. These cells 
bear rods which are remarkably like those of Peripatus and 
have the same marginal (lateral) ‘Stiftchensaum’. Indeed, 
the rods of the Scolopendra retina stain very lke those of 
Peripatus. 
Hesse speaks of the retinal elements of these eyes as being 
of a very original type. It is particularly interesting, therefore, 
to find the agreement with Peripatus. 
The histology of the Polychaete eye has been investigated 
in some detail by R. Hesse (8). We can find material for 
eomparison in his papers. 
