172 
THE VOYAGE OF H.M.S. CHALLENGER. 
brain, optic nerve, and retina all in one. We shall return to its further consideration 
presently. 
Immediately within the pigment is a clear, firm, faintly granular material, exhibiting 
no definite structure, and occupying the posterior two-thirds of the eye-cavity. Although 
corresponding in position within the rod-layer in the Alciopod’s eye, yet from its general 
appearance and absence of structure it is in all probability only vitreous, so that the rod- 
layer of the retina is quite unrejDresented in the eye of GenetylUs lutea. In one section 
only an appearance suggesting the rods was seen, but this may possibly have been due to 
fine folds (see fig. 6, a). In many of the sections the entire pigment-layer with clear 
substance still adhering to it has become detached from all its other connections 
(PI. XXXIIIa. figs. 6 and 8). 
In one section there is a deposit of a coarsely granular, coagulated substance resting 
in the form of little heaps on the inner surface of the vitreous. It has no definite shape, 
but suggests, from its general appearance and position, a refractive substance of the 
nature of a lens. Possibly it is semifluid during life. 
A horizontal section through the middle of the fundus of both eyes enables us to 
understand the relations of the nervous matter in this situation. We then observe that 
there is one large medial ganglion, having a large lateral lobe on each side projecting, as 
above mentioned, into the fundus of the corresponding eye. The central or medullary 
portion of the ganglion consists of molecular material and small nucleated cells, while at 
the periphery or cortex there are large, well-formed ganglion-cells with distinct nuclei and 
occasionally indications of nucleoli (PI. XXXIIIa. fig. 1, e.). On employing high powers, 
very delicate fibrils are visible at the junction of the cortical and medullary portions, 
which seemingly run from one lobe to the other. As will be readily understood from 
the bilobular shape of the ganglion, it is only from sections through the immediate 
neighbourhood of its centre that its structure can be duly appreciated. The nearer we 
approach the periphery in our sections, the more we have the appearance of two distinct 
ganglia. Thus in a section through the anterior part of both eyes, we have a thick 
fibrous structure dividing the two lobes, directly continuous with the previously described 
junction of the sclerotic coats of the two eyes. This ganglion must be regarded as a 
fused condition of the two cephalic ganglia normally present in this position in the 
Annelida. 
The vessels seen in sections of the sclerotic in the median line are disposed differently 
in different preparations. In sections nearly through the centre of the globes, we find 
one or two small oval spaces on each side of the middle line (PL XXXIIIa. fig. 1, a). 
If the anterior part be divided, however (PI. XXXIIIa. fig. 4), we find only one space 
corresponding to a median vessel cut across. Thus probably the dorsal median vessel 
splits into two or more lateral branches over the prominence of the inner sides of the 
eyeballs, to meet again anteriorly and form a ventral median vessel. 
