RELATION OF MEDIAN TO LATERAL EYES 353 



between the higher vertebrate classes and either of the two invertebrate 

 phyla mentioned. At the same time a comparison of the very early stages 

 of development does indicate a descent of these highly differentiated and 

 specialized types from an extremely remote common stock, of which we 

 have no geological record, but which it may be supposed existed as a 

 simple form of animal adapted for life in water or in the mud or sand at 

 the bottom of the sea, fresh water lakes, or estuaries of rivers. We may 

 accordingly examine such types as are living to-day under similar con- 

 ditions. As examples we may mention such animals as Planaria or simple 

 forms of Annelids ; in these we find a type of sense-organ capable of 

 reacting to light in an animal which shows bilateral symmetry of the body 

 generally and more particularly of the central nervous system, e.g. as 

 in Planaria or the Annelid Hcemopis sanguisuga (Fig. 16, Chap. 3, p. 21). 

 The essential elements for the higher types of eye are present in these 

 simple forms, namely : pigment, a refractive medium, and neuro-sensory 

 cells connected by nerve-fibres with the central nervous system ; we see 

 also in Hcemopis the commencement of the differentiation of a pair of 

 aggregate lateral eyes, and between these two simple eyes formed by the 

 modification of a single cell. It may be conceived that the lateral paired 

 eyes, each consisting of a group of modified cells and being situated in a 

 more favourable position than the median eyes, would in the process of 

 time become still more differentiated, and that in place of single receptive 

 or neuro-sensory cells containing pigment in one part, a clear refractive 

 mucoid material in another, and terminating in a nerve-fibre, which 

 originates from its deep aspect, special pigment cells and refractive cells 

 would be set apart : the pigment cells for the absorption of superfluous 

 rays of light, the clear refractive cells for the dioptric mechanism, e.g. 

 " lentigen cells " and " cone cells," while others will serve as retinal or 

 receptor cells and still others for the conducting of visual impulses through 

 one or more optic ganglia to the brain. 



As the requirements became greater, still further adaptive changes 

 ensued ; thus a hard protective covering or scleral tunic was developed 

 around the sensitive epithelial retina ; muscles were evolved to enable 

 independent movements of the eyeball to take place ; also an outer 

 chitinous or cartilaginous capsule which enclosed an orbital cavity, and 

 which at the same time provided a framework for the attachment of the 

 specialized orbital muscles (Figs. 91 and 92, Chap. 11, pp. 129, 130) in 

 some instances, e.g. in Sepia (Fig. 36, Chap. 3, p. 50, and Fig. 121, Chap. 12, 

 p. 164), a movable iris-like fold and intraocular muscles were developed 

 for regulating the amount of light entering the eye, and possibly accom- 

 modation of the lens for near or distant vision ; also a " secondary cornea " 

 growing as a fold over the front of the eyeball and enclosing a chamber, 



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