CHAPTER II 



General Characteristics of the 

 Sensory System 



One of the biggest challenges that animals faced in their evolutionary 

 history occurred when they essayed terrestrial life. By all counts the 

 sea is a more permissive environment than land. It is above all a 

 more stable and uniform environment. It exhibits no profound 

 temperature changes; humidity is no problem; osmotic relations are 

 constant. Consequently, the need to develop sense organs to detect 

 changes in these realms is minimal. The physical properties of water 

 obviate the necessity of differentiating between olfaction and taste 

 in the sense that terrestrial organisms do. Density precludes distant 

 vision as well as limiting wavelength discrimination. Since water is a 

 medium of transport, food procurement does not present all the 

 problems that confront the terrestrial animal. By the same token 

 water serves as a medium of dispersal of eggs and sperm and as a 

 cradle for the young so that the complex sensory discrimination and 

 behaviour patterns which have evolved among land animals for 

 reproduction and parental care are largely absent in the sea. 



One of the more fundamental problems that confronted animals on 

 emergence to land was that of support, since air does not lend the 

 helping hand that water does. Before emerging on land, animals had 

 already set out on two paths of skeletal development. The arthropods 

 cast the die for an exoskeleton; the chordates, for an internal 

 skeleton. The choice of skeleton had profound effects upon the 

 direction that the development of the nervous system followed. The 

 skeleton is a major limiting factor; all other organs accommodate to 

 it. The arthropod exoskeleton determined the method of growth - the 

 only way to increase size is by moulting - and it also limited the overall 

 size of the animal. Once free of the support of the sea, the animal was 

 limited in size by the engineering principles of a frame dwelUng (cf. 

 Thompson, 1943). This may be one of the reasons why insects in 

 general are small animals (although during the Carboniferous one 

 dragonfly attained a wingspread of more than 2 ft.). The largest living 

 species are somewhat larger than the smallest mammals, while the 



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