48 
PROFESSOR H. N. MOSELEY. 
the optic nerves are distinguished from the strands supplying 
the megalaesthetes by being slightly pigmented for a consider- 
able extent of their course. A large proportion of the eyes in 
Ornithochiton are supplied by nerves which are given off from 
the soft tissue strands entering the shell along the sutural lines, 
but many eyes are also certainly supplied by pigmented strands, 
which can be traced only to the free margins of the tegmenta 
adjoining the girdle. In those shells in which only single rows 
of eyes are present coincident with the sutural lines, the eyes 
seem to be all supplied by strands passing from the sutural 
line and specially ramifying in order to reach them. Within 
the pigmented tubular prolongation of the eye capsule the 
numerous fine fibres composing the optic nerve become sepa- 
rated from one another and loose. Immediately beneath the 
retina the fibres become still more widely separated, forming an 
expansion of fibres. The retina is formed on the type of that 
of Helix, and not, as might have been expected, on that of the 
dorsal eyes of Onchidium or the eyes of Pecten. The fibres 
of the optic nerve do not pass in front of the layer of rods to 
be distributed to them from in front, but are directed to the 
rods directly from behind. The retina presents a single layer 
of short but extremely well-defined rods (PI. VI, figs. 6, 7), 
the extremities of which are directed towards the light. 
The rods, when viewed from the surface of the layer they com- 
pose, are seen to be hexagonal or pentagonal in outline, and 
each contains a nucleus. They form a layer which is concave 
towards the lens, there being a space between the hind surface 
of the lens and the concave face of the layer. 
The rods closely resemble in appearance those figured by 
Semper as occurring in Onchidium. Immediately beneath the 
rod layer is a stratum or several layers of nuclei amongst the 
ramifications of the nerve-fibres. The structure of the retina^ 
as described, has only been made out in specimens ofAcantho- 
pleura spiniger, which alone of the material available were 
in a condition of preservation sufficient to permit it. Similar 
expansions of the optic nerve have been seen, however, to occur 
in many other forms. 
