CHAPTER 24 



RELATION OF THE MEDIAN 

 TO THE LATERAL EYES 



In considering this question it will be necessary first to review briefly 

 the development and structure of median and lateral eyes in the simplest 

 forms of invertebrates, and to trace in living representatives of phyla in 

 which bilateral symmetry has been evolved, the gradual differentiation 

 and perfection of the paired lateral eyes of the higher types, in association 

 with habits of life requiring accurate vision and quick perception, as for 

 instance in the dragon-fly or cuttle-fish ; and to contrast the eyes of these 

 with the type of eye which suffices for their free-swimming larvae, or the 

 adult animal of lower types which have not advanced much from the 

 larval form, and in which the adult creature lives in an environment 

 similar to that of the larva, e.g. as in some of the non-parasitic Turbellaria. 

 It will be necessary also to consider the conditions of life, which it 

 may be assumed existed in the earliest stages of the ancestral history or 

 phylogeny of these groups ; that is to say, before a terrestrial existence 

 was possible or an active predatory and carnivorous life could have been 

 maintained, either in the open sea, on land, or in the air. Such a life is 

 possible only in animals with very highly evolved sensory organs, central 

 nervous system, and organs of locomotion, such as fins, jointed-limbs, or 

 wings. A comparison also may be made between the sense-organs of the 

 more highly evolved types and those which live a burrowing life in the 

 mud at the bottom of shallow waters, such as in ponds or tidal waters 

 round the coast, or of those which lead a parasitic existence. If this is 

 done, it will at once become evident that enormous periods of time must 

 have elapsed in order to allow the evolutionary changes which have 

 occurred in the higher types of animals to take place, and also that in 

 tracing back the origin of any particular phylum, it is not the adult species 

 of the more highly differentiated types of the orders that are likely to 

 prove of value in assessing the relationship of different groups, but the 

 larval forms and more particularly those species in which the larvae 

 have a free-swimming existence in the water or on moist herbage. These 

 considerations limit the extent of the inquiry very considerably, and 

 imply that there can be no very close relationship between such divergent 

 and specialized types as those of the higher Crustacea and Mollusca ; or 



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