102 PATTEN. [Vo.. II. 
eye, and there is the same absence of nuclei beneath their 
cuticular thickenings. I have not been able, however, to de- 
tect any evidence that they are supplied with separate bun- 
dles of nerve fibres, but this will not seem strange, when we 
consider their small size and transitory nature. I see no 
reason why they, like the sensory patches composing the cen- 
tre of the eye, should not be regarded as remnants of dis- 
tinct sense organs. This supposition will appear in a more 
favorable light, perhaps, when we consider similar patches 
connected with the rudiments of the remaining eyes. 
The sensory spots, or primitive eyes, of which the clear area is 
composed, are comparable with those on the mantle edge of Avca, 
as described by me in “ Eyes of Molluscs and Arthropods” (PI. 
30, Fig. 42). It was there shown that some of the eyes arose as 
simple, pit-like depressions covered by cuticular thickenings, into 
which the everywhere-present intercellular nerve fibres extended, 
producing the appearance of vertical striations. Such eyes con- 
tain the lowest stages in the development of visual rods. In more 
specialized eyes it was shown that the cuticula had broken up 
into blocks, one overlying each cell. The nerve fibres arrange 
themselves around these blocks in various ways, and a true 
retinal rod is the result. 
In the embryos of Acz/ius, the eye passes through the stages 
permanently represented on the mantle edge of Molluscs, for 
the cuticular thickening over each optic pit, at first fuzzy and 
non-refractive, soon changes into a layer of stiff and refractive 
cilia-like bodies, which finally form a dense and almost homo- 
geneous cuticular layer. As soon as the retinal cells become 
distinctly outlined, this cuticula breaks up into a number of 
minute rods, two being formed over each retinophora. The 
arrangement of the nerve fibres about the rods will be de- 
scribed later. It is sufficient for the present to state that it 
corresponds in almost every particular with that found in the 
more highly developed rods of Molluscs. Moreover, in Acz/zus 
as in Molluscs the cuticula overlying the sensory cells is divided 
into two layers: a thin outer membrane devoid of nerve fibres, 
the corneal cuticula ; and a thicker, inner one, the retenzdial cute- 
cula, so called because it contains nerve ramifications, and gives 
rise to the rods. The development of the rods in Acilius, there- 
fore, is in perfect harmony with the view concerning the phylo- 
