THE EYE-LIKE SPOTS IN FISHES. 
231 
The mother-of-pearl organs (PL YI. fig. 4) are, as the figure 
shows, appropriately named ; they have in all cases an outer 
brown investment, a metallic layer, a gelatinous internal body 
formed of connective tissue ; they are provided with nerves and 
blood-vessels and are surrounded by a lymphatic space. The 
metallic plates are regarded by Ussow as special cells, but 
Leydig looks upon them as altogether similar to the minute 
iridescent bodies found on the skin. The gelatinous portion is 
made up of delicate radiate cells, which give rise to a network, 
and an intermediate soft substance. No essential difference in 
structure is presented by the so-called luminous organs. 
If we look at the bodies from the point of view suggested by 
Prof. Leydig, it is impossible not to assent to his conclusion 
that, whatever they are, they are not sensory in function. At 
first he was inclined to regard them as belonging to that system 
of sensory parts which runs down either side of a fish’s body, 
and which is seen from without as a well-marked ‘lateral line,’ 
but this was a doctrine which a further examination showed 
him to be utterly untenable. 
After these investigations, Leydig proceeds to sum up his 
objections to the eye-like organs being regarded as presenting 
anything truly characteristic of the animal eye ; and it must be 
allowed that he makes some of his points with very great force 
indeed. The examination of Chauliodus alone might leave the 
investigator with a mind inclined towards the explanation given 
by Ussow, but a clearer view is gained by testing the doctrine by 
an investigation into the organ of Argyropelecus ; here (fig. 3) 
we see that the ‘ retinal 5 nerve-cells are connected with the in- 
vestment formed by the layer of connective tissue ; that in fact 
they are supporting rather than nervous fibres. So again, the 
brown pigment at first calls to mind an optic organ, but this 
brown pigment is more widely distributed through the body of 
the fish, and especially is it developed in the ‘ frontal gland ’ 
or epiphysis of the brain, where also it leaves a central, clear, 
un pigmented spot. Nor is this all, — the pigment, as our figure 
shows, is not laid down continuously, as it is in a choroid, but is 
scattered at various points. The Bonn histologist carries his 
merciless criticism yet further; the so-called ‘lens,’ which so 
easily has ascribed to it the refractive function, does not lie in 
the centre but at the edge of the mass w T hich does duty as the 
vitreous body, and, in addition to its position, its histology pre- 
vents us from regarding it as a lens. 
One possibility remains ; it may be argued that the bodies 
in question are not to be compared to the ordinary vertebrate 
eye, but that they are nevertheless terminal sensory organs 
affected by light ; but here, unfortunately, we come upon 
