THE EYE OF PECTEN. 
449 
this condition occurs especially in the rods at the side con- 
vexities. Fig. 6 represents an isolated rod in this con- 
dition. 
The anterior ends of the rods contract considerably, and 
again expand into spindle-shaped bodies, each of which con- 
tains a nucleus ; so that in P.jacobceuSy where the retinal 
elements of this region are difficult to distinguish, there 
may be seen a single row of nuclei running from end to end 
of the retina, and following its sinuosities (Plate XXXV, 
fig. 115). 
In some of the rods at the side of the retina a second 
spindle-shaped body follows the first one, as represented in 
the isolated rods in figs. 5, 6, but usually the anterior end 
of the spindle is drawn out into a delicate thread, which 
occasionally possesses nuclear swellings. Finally, this thread 
breaks up into a network, which bears a number of nuclear- 
like bodies at its nodes, and several round molecular bodies 
appear to be caught in its meshes. These bodies are so 
much like the ordinary nuclei of the network that I am in- 
clined to believe that they are, in reality, merely modifica- 
tions of them, and in some way connected with the network 
(fig. 6 a). Anteriorly the fibres of the network bend at right 
angles and enter the nerve layer, which covers the anterior 
surface of the retina. This nervous layer will be described 
with the description of the optic nerves. 
The above is a description of the retina as I found it in 
P. maximus, and I believe it holds good for the other mem- 
bers of the genus. The elements of the retina are so much 
larger in this species, and the spaces between the rods and 
network, &c., so much more considerable, that it is a great 
deal easier to investigate ; but I believe careful examination 
of the other species would show that they do not differ from 
this in any important detail. 
The tapetum — is placed immediately behind the retina, and 
may help in its support. When fresh,^ the tapetum exhibits 
a display of colours, and it is this membrane which gives 
the eyes their beautiful metallic lustre. When examined 
with a 4th-inch obj. it seems to be composed of a great 
number of little black specks separated by a fine yellow 
membrane, but careful examination with a higher power 
shows that it is composed of a great number of fine fibrils 
crossing at right angles. 
The space between the tapetum and the posterior part of 
‘ 1 have one series of seetions stained in osmic aeid, and mounted in 
Canada balsam, whicli lias retained this display of colours. 
