8 SYDNEY J. HICKSON. 



hardened by alcohol or other reagents the pigment adheres 

 to the tapetum or posterior wall 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 nertous 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, w^hich runs round 

 the margin of the mantle, and which Duvernoy calls the 

 " circumpaliaF^ nerve. This nerve is figi:red in section in 

 fig. 1, PI. I, one of the nerves joining this nerve being 

 figured at fig. 1, q. This " circumpalial" 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 II, 

 fig. 9, of P. maximus, shows the division of the single nerve 

 into its two branches. In fig. 1 the course of the optic nerve, 

 before its division into two branches, was carefully drawn 

 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, which supply the tissues 

 surrounding the eye. The course which the retinal 

 branch takes may be seen in PI. J, fig. 1, and in PI. II, 

 figs. 8 and 9. In figs. 8 and 9, the first section is cut through 

 the optic nerve, and shows the manner in which the retinal 

 branch runs up the side of the eye-cavity ; the second section 

 shows the manner in which the branch bends over on to the 



