450 
SYDNEY J. HICKSON. 
the eye-cavity is filled with a red fluid pigment. In the fresh 
condition the pigment readily flow's on to the slide when the 
eye is pricked, but in sections of the eye which has been 
hardened by alcohol or other reagents the pigment adheres 
to the tapetum or posterior w'all of the eye-cup. 
Hensen figures a layer of cells in this position, but I have 
never been able to observe anything of the kind ; the pigment 
contains no cellular elements at all, nor is there a layer of 
cells lining the cavity which contains the pigment. The 
pigment consists of a number of bright red granules floating 
freely in a colourless fluid. 
The nereous supply — of the eye of Pecten is perhaps the 
most interesting of the many peculiarities of this eye. The 
nervous system of Pecten is well described by Duvernoy in 
the paper referred to above. The mantle is supplied by a 
number of branches given off from the principal ganglia. 
These branches all fall into a large nerve, which runs round 
the margin of the mantle, and which Duvernoy calls the 
circumpaliaP^ nerve. This nerve is figured in section in 
fig. 1, PI. XXXIV, one of the nerves joining this nerve being 
figured at fig. I, q. This circumpaliar^ nerve gives off 
filaments to supply the tentacles and eyes. 
Krohn first gave a drawing of the optic nerve, and described 
it as a single nerve passing off from this trunk, and dividing 
into two branches as it aproaches the eye. Later observers 
have, however, drawn and described two nerves passing off 
from the circumpalial” nerve. My researches have led 
me to believe that Krohn is right, and that such a figure as 
Hensen gives in his paper, representing two main trunks 
passing up to supply the eye is erroneous. Plate XXXV, 
fig. 9, of P. maximuSi shows the division of the single nerve 
into its two branches. In fig. I the course of the optic nerve, 
.before its division into two branches, was carefully drawm 
from one of a complete series of sections, and in none of the 
other sections could I find a trace of any other nerve pro- 
ceeding from the circumpalial.” The branching of the 
nerve takes place in a plane at right angles to the plane of the 
mantle. When the optic nerve approaches the eye it divides 
into two branches, which may be called the ‘^retinal nerve” 
and the complementary nerve.” The former passes up the 
side of the eye cavity, and spreads over the anterior surface 
of the retina ; the latter soon loses its sheath, and divides 
up into a number of branches, w'hich supply the tissues sur- 
roundiijg the eye. The course which the retinal branch 
takes may be seen in PI. XXXIV, fig. 1, and in PI. XXXV, 
