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After having given a sketch of the principal parts of the 
human eye, Mr. Rean observed that at the very outset of our 
investigations a difficulty confronted us. What is the lowest 
form in which any organs presented themselves for transmitting 
the rays of light? There are many creatures which are either 
attracted or repelled by the rays of light, and yet afford no indi- 
cation to the most careful examiner of any optical apparatus. It 
may be that the heat which accompanies the ray acts on some 
nerve sensitive to its action. Probably in the Rotifera and 
Entozoa some appreciation of light exists, though no organ has 
been discovered specially adapted to its reception. 
In the Annelids the organ of vision undoubtedly exists as 
simple globular prominences in the skin. In the leech there are 
10 in a semi-circle round the front of the mouth, in other worms 
there are four. In these there is a transparent skin which may 
be regarded as the simplest form of the cornea. 
Among the Molluscs, the Gasteropoda shew eyes far more 
developed than in the Annelids. 
In the common snail they are at the extremity of the so called 
horns. They consist of a transparent portion of skin, answering 
to the cornea, a tubercular extension of the optic nerve, containing 
a black choroid and in front a perfectly developed globular 
lens. Passing to the Articulata, one is overwhelmed by the 
delicacy aud beauty of the various forms of optical apparatus. 
The eyes of insects may be divided into four distinct classes, firstly, 
The simple eye ; secondly, the conglomerate, composed of many 
simple eyes aggregated together ; thirdly, the compound, formed of 
numerous tubes, each tube composing a separate optical instru- 
ment, and fourthly, the apparently compound eye, but one with a 
single cornea covering the whole series of tubes. Some insects 
possess two of these varieties. 
In the Crustacea, the eyes are situated on a longer or 
_ shorter pedicle, capable of movement in any direction, and may 
be withdrawn within a sheath if danger threatens, The eyes of 
