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A THREE-EYED REPTILE. 
lias found that it has all the structure of a perfect eye—that 
is, lens, choroid, and retina are present, the nervous elements 
of the latter being connected with the brain by nerve-fibres. 
All stages between this perfect state and the ordinary form of 
a mass of simple tissue have been discovered by Graaf and 
Mr. Spencer in various lizards and amphibia, and conse¬ 
quently the “ pineal gland ” is proved to be a median eye. 
It is, however, important to note that this median eye 
differs essentially from the ordinary lateral eyes of Vertebrates 
in the arrangement of the layers of the retina. Now this 
structure is composed essentially of a layer of nerve-fibres, 
which unite at one point to form the optic nerve, and of a 
layer of minute bodies called rods and cones. The latter are 
connected with the nerve-fibres, and by them the energy of 
light waves is converted into a form capable of affecting 
nervous tissue. In the vertebrate type of eye the former of 
these two layers lies between the light and the rods and cones ; 
but in the invertebrate type the position is reversed. This 
difference between the eyes of the two great classes of animals 
is due to their radically different modes of development, a 
subject of some difficulty, which need not be treated in a 
popular paper. Briefly, the nervous part of the vertebrate 
eye is developed as a hollow outgrowth from the brain, which 
is not the case in the Invertebrates. 
The median eye of Hatteria has its layer of rods and 
cones placed between the light and the layer of nerve-fibres, 
and in this respect resembles the invertebrate type, but since 
it is formed as an outgrowth from the brain, we must regard 
it as really developed on the vertebrate type. 
Now the presence in a Vertebrate of a third eye is 
sufficiently interesting, but beyond this it confirms certain 
theories respecting the ancestry of the Vertebrates, and it is 
here that the importance of the discovery lies. To appreciate 
it, however, we must consider a group of animals much lower 
in the scale of organisation. 
There may often be found attached to the rocks and stones 
of our sea shores, when uncovered at low water, lumps of 
greyish, leathery substance, irregular in shape and size, which 
the casual observer would scarcely imagine to possess life, and 
certainly would not credit with being animals. If, however, 
one of them be roughly touched, a stream of salt water 
suddenly ejected into his face will disabuse his mind on these 
points, for this “ Sea-squirt,” or more scientifically, Ascidian , 
holds a high place in the animal kingdom, and belongs to that 
great division of it which is characterised by possessing a 
back-bone. This amazing fact has been ascertained by 
