SIGHT. 593 



have eyes, numerous or single, single originally, or several united into one, but lying 

 flat on the surface, sessile. So with most free annelida : in the planaria viganensis 

 there is a single row of about forty eyes. In addition to the pigment and nerve 

 there is a lens. In different species of gasteropoda, as the slug, snail, limpet, eyes 

 are situated at the base, middle, or extremity of their tentacula. The eye of the 

 helix pomatia has within its pigment a transparent semifluid substance, and even 

 another transparent body shaped like a lens. The murex tritonis and voluta 

 cymbrium have also an iris forming a pupil. Among the articulated animals 

 we find the lucid spots already mentioned, called also simple eyes and stemmata ; 

 conglomerate eyes, or clusters of these ; and compound eyes, or a large assem- 

 blage of small tubes each with its own humours, retina, and cornea ; together 

 with eyes supplied each with separate lenses and retinae, but having one common 

 cornea. Among myriapods we have examples of the conglomerate eye ; in the 

 scolopendra there are twenty contiguous circular lenses arranged in five lines, 

 with one larger eye behind the rest, like a sentinel ; in the millipede, there are 

 twenty-eight, arranged in a triangle of seven rows, each having one eye fewer 

 than the row above. In the larva state insects have only stemmata if they are 

 destined for a complete metamorphosis ; but, if to an incomplete, they have both 

 compound and simple eyes. Some zoophytes, being free in one state have eyes, 

 which they lose on being metamorphosed and no longer free, when eyes would 

 be useless. Except some parasitic insects and five species of ants, all insects 

 have compound eyes, generally one on each side of the head, forming a 

 globular mass of from fifty to some thousands, even twenty or thirty thousand 

 minute eyes closely pressed together, and placed on a central bulb which is a 

 part of the optic nerve. They are cylinders or cones, while their external part 

 or cornea is the base, usually hexagonal, like the cells of a honeycomb, because 

 this form allows uniformity of arrangement with the greatest economy of space. 

 The united bases or corneae are an hemispherical convexity ; under each cornea 

 is an almost conical lens. Each cornea is covered by smooth epidermis, just as 

 our cornea is covered by conjunctiva, with its apex backwards towards the nerve, 

 and a portion of chorioid pigment lies between the cornea and lens, with an 

 aperture in the centre, constituting a rudimentary iris, which in the grey dragon 

 fly has been seen to contract and dilate ; and between it and the cornea are a 

 little space and a drop of aqueous humour. The pigment runs backwards 

 around the lens, separating each cylindrical compartment. The apex of the lens 

 is met by the end of a filament of the optic nerve, running through a vitreous 

 humour ; and Dr. Wollaston found the focal distance to correspond accurately 

 with the length of the tube, so that an image falls exactly upon the retina. These 

 multiplicities of corneae in all directions compensate for the want of sensibility in 

 the eyes of insects. There are often simple detached sessile eyes also, equally mo- 

 tionless : and the purpose of the presence of both kinds is unknown. There is 

 no apparatus of defence, no eyelids, eyelashes, or tears, except that hair some- 

 times grows from between the corneee ; and these are compensated for by the 

 great hardness and insensibility of the cornea, which allows it to be brushed 

 with the hairs of legs or other moveable parts. Branches of a trachea have 

 been traced into each kind of eye. In the arachnida the optic nerve expands 

 into a cup-like form behind a hyoloid membrane and vitreous humour. 



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