EEPORT ON THE ANNELIDA. 
181 
are several rods inserted into the same cell-area. In figs. 14 and 21, moreover, we find 
large light spots in the pigment, neither in size nor position suggesting sections of the 
rods, which in both cases are narrow. Here I believe we really have nuclei, and 
in fig. 18 one or two of the nuclei figured also appear to belong to the pigment- 
layer. In PL XXXIVa. fig. 1 we have a most remarkable appearance, part of this 
layer being practically devoid of pigment.^ In this colourless spot we find a small 
nucleus, which would doubtless have escaped observation in the normal pigmented 
condition. 
Immediately outside the pigment we have a layer of nucleated columnar cells, vary- 
ing in length according to their situation, becoming shorter towards the periphery of the 
retina. The inner end of each is, as it were, crowned by a deposit of the pigment-layer, 
while the outer end tapers off and is continued as a nerve-fibre. I have never seen proof 
of a direct connection between this cell and the outer end of a rod. If such a junction 
exist, as is supposed by Greeff, it is concealed by the position of the pigment deposit. 
The large globular masses of pigment near the periphery of the retina have well-marked 
columnar cells connected with each (PI. XXXIIIa. figs. 11, '"16, 17, 18), but these do not 
appear to have any connection with the layer of nerve-fibres. Where there are no darker 
aggregations in the pigment-layer, as described above (figs. 16, 18), we find nuclei but 
no columnar cells. At intervals among the columnar cells we find clear, highly refracting 
tapering bodies, each resting by its base on the pigment-layer. Their fine terminal 
processes can be traced quite half-way through the columnar layer. I consider them to 
be of the nature of a supporting connective-tissue like the Mullerian fibres of the verte- 
brate retina (PI. XXXIIIa. fig. 16; PI. XXXIVa. figs. 1, 3, 4). They are possibly 
processes (?) of the cellular membrane forming the pigment-layer. 
Measurements. 
Diameter of lens, ..... 
0-32 mm. 
Length of rods, ..... 
0'028 to 0'07 mm. 
Breadth of rods, ..... 
. 0'004 to 0'012 mm. 
Thickness of cortex of rods. 
O’OOOS to 0'0012 mm. 
Average thickness of pigment-layer at insertion of rods, 
(about) 0'008 mm. 
Pigment-aggregations of rods, .... 
0-0045 mm. 
Thickness of peripheral pigment-layer, . 
0’02 mm. 
Diameter of large peripheral pigment-balls. 
0-012 mm. 
Ultimate pigment-granules forming latter, . 
0-001 X 0-0005 mm. 
Length of columnar cells of retina proper. 
. 0-06 to 0-1 mm. 
Length of columnar cells near periphery. 
(about) 0-06 mm. 
^ Several sections passing through this non-pigmented area were cut consecutively and lie in proximity on the same 
slide. Of all the sections of the retina of species of Alciopa that I have made and examined, this is the solitary example 
of such a condition, and I am inclined to think that we have really here to deal with a congenital abnormality, occur- 
ring in this individual. 
