14 
PROFESSOR H. N. MOSELEY. 
thè optic nerves are distinguished from thè strands supplying 
thè megalsesthetes by being slightly pigmented for a consider- 
able extent of their course. A largo proportioa of thè eyes in 
Ornithochiton are supplied by nerves which are given off from 
thè Soft tissue strands entering thè shell along thè suturai liues^ 
but many eyes are also certainly supplied by pigmented strands, 
which can be traced only to thè free margins of thè tegmenta 
adjoining thè girdle. In those shells in which only single rows 
of eyes are present coincident with thè suturai lines, thè eyes 
seem to be all supplied by strands passing from thè suturai 
line and specially ramifying in order to reach them. Withiu 
thè pigmented tabular prolongation of thè eye capsule thè 
numerous fine fibres composing thè optic nerve becorae sepa- 
rated from one another and loose. Immediately beueath thè 
retina thè fibres become stili more wùdely separated, forming an 
expansion of fibres. The retina is formed on thè type of that 
of Helix, and not, as might bave been expected, on that of thè 
dorsal eyes of Onchidium or thè eyes of Pecten. The fibres 
of thè optic nerve do not pass in front of thè layer of rods to 
be distributed to them from in front, but are directed to thè 
rods directly from behind. The retina presents a single layer 
of short but extremely well-defined rods (PI. VI, figs. 6, 7), 
thè extremities of which are directed towards thè light. 
The rods, when viewed from thè surface of thè 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 thè lens, there being a space between thè liind surface 
of thè lens and thè concave face of thè layer. 
The rods closely resemble in appearance those figured by 
Semper as occurring in Onchidium. Immediately beneath thè 
rod layer is a stratum or several layers of nuclei amongst thè 
ramifications of thè nerve-fibres. The structure of thè retina, 
as described, has only been made outin specimens of Acantho- 
pleura spiniger, which alone of thè material available were 
in a condition of preservation sufficient to permit it. Similar 
expansions of thè optic nerve bave been seen, however, to occur 
in many other forms. 
