CHAPTER 8 



SENSORY ORGANS AND RECEPTION 



Sense which abroad doth bring 

 The colour, taste, and touch, and scent, and sound, 

 The quantity and shape of everything. 



Sir John Davis, Nosce Teipsum, 1 599 



INTRODUCTION 



All animals are sensitive to certain environmental changes which act as 

 stimuli, influencing the behaviour of the organism as a whole, or of its 

 component parts. Although sometimes directly exciting effector elements, 

 external stimuli usually act on sensory structures, which in turn feed in- 

 to the nervous system the information received. The relative importance of 

 the several sensory modalities in the economy of animals varies greatly. 

 In the euphotic and littoral regions of the sea, light, as an environmental 

 stimulus, has an importance equal to that of other sensory cues in regula- 

 ting the overt behaviour of the animals found there. But nocturnal animals, 

 and those found in aphotic zones of the ocean, may depend largely or 

 exclusively upon other sensory modalities for information about changes 

 in the external world. 



Irritability is a generalized property of protoplasm, as seen in the res- 

 ponses of protozoa and sponges to various kinds of stimuli. In metazoans 

 effective stimulation involves excitation of the nervous system. The stimuli 

 may affect free nerve endings in the skin, or may act on peripheral receptor 

 cells. The epidermis of primitive animals contains undifferentiated primary 

 sense cells, probably responsive to a wide range of stimuli (Figs. 8.1, 

 8.2). Frequently, however, the receptors are differentiated so as to possess 

 maximum sensitivity to some particular kind of stimulus. This process of 

 differentiation and specialization reaches its apogee in image-forming 

 eyes and in the ear of higher vertebrates. 



PHOTORECEPTION 



In lowly-organized forms, such as amoebae, the general protoplasm is 

 photosensitive. Higher animals frequently possess specific photoreceptors 

 which take the form of scattered light-sensitive cells, or more highly 

 constructed multicellular organs of varied grades of complexity. These, 

 according to the level of intrinsic organization and the complexity of 

 central nervous pathways, are utilized for detecting intensity differences, 

 direction of light rays, quality of light (spectral composition), form and 

 pattern, the latter by ocular organs of considerable complexity. More 

 than any other sensory guide, light regulates the orientation of animals, 



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