PRESENCE OE EYES IN SHELLS OF CERTAIN CHITONIILE. 49 
The only pigment present in the eyes examined is that by 
which the eye capsules are rendered opaque. No pigment seen 
in connection with the rods or in connection with the nervous 
elements. Possibly the absence of such pigment is due to the 
imperfect preservation of the material. 
Not all the fibres of the strand entering the eye cavity 
proceed to the retina. A large number of peripherally placed 
fibres pass outside the retina all round, and, passing through 
apertures in the iris at its outer margin, end at the surface of 
the shell all round the area occupied by the corneae. They 
terminate in micraesthetes exactly corresponding in structure 
to the other micraesthetes present and identical with them in 
structure. 
They apparently form a sensitive zone round each eye, and 
their strands arise from the optic nerve just as do those of many 
of the other micraeesthetes from the megalaesthetes (see PI. YI, 
fig. 6, b' b' ; fig. 4, b ; PI. Y, fig. 8, b' b'). On their way to the 
surface these strands, given off by the optic nerves to the 
micraesthetes, traverse a series of slit-like perforations of the 
ris, which are conspicuous, and at first very puzzling features 
in the iris structure when the eyes are viewed in the decal- 
cified tegmentum from its external surface by transmitted 
light (PI. VI, fig. 5, b b). 
In the eyes of some forms when thus viewed, an open fold 
or gutter leading from the bulb superficially along the stalk 
of the pear is seen, curiously recalling the choroid fissure 
(PI. VI, fig. 5). The occurrence of double eyes, combinations 
of two eyes fixed closely side by side with a common nerve 
stalk, is not an uncommon mode of growth. 
Growth of the Tegmentum, Eyes and ^Esthetes. 
The tegmenta increase in growth by additions formed at 
their margins where they adjoin the girdle regions of the 
mantles. The additions are probably made by the mantle 
tissues which immediately abut on the tegmental margins. 
At the basal margins of the tegmenta of all forms in which 
VOL. XXV. NEW SER, 
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