212 The Vertebrate Organ Systems 



central nervous system through afferent nerves of the peripheral system. 

 The endings of these nerves are variously specialized to respond to 

 changes of certain definite stimuli ; for example, the endings within 

 the eye respond to light changes, those in the tongue to taste changes, 

 and some within the skin to pressure changes. 



All animals respond to changes within their environment to vary- 

 ing degrees. Even among the protozoans, definite responses to light, 

 temperature, and chemicals can be noted. For most protozoans, there 

 do not seem to be specialized structures for the detection of these changes, 

 rather the whole surface seems to respond. There are, however, in- 

 numerable specialized structures among the various invertebrate groups. 

 Some examples are the special balancing organs of the jellyfish, the 

 light sensitive spots at the tips of the arms in the starfishes, the tactile 

 "ears" of the flatworm planaria, the antennae and complex compound 

 eyes of the insects, and the vertebrate-like eyes of the squid and octopus. 



Among the vertebrates the sense receptors are developed to a very 

 high degree for the detection of environmental changes of all types. 

 Essentially all these receptors are but nerve endings, but many have 

 complex accessory structures to add to their efficiency. We can only 

 understand the functions of these organs in terms of what we ourselves 

 experience ; thus it is often difficult to understand the exact nature of 

 some special sense organ in another group of animals. One example 

 of this is the interpretation of the exact nature of the lateral line sense 

 organ of many fish. Man has no sense organ which corresponds exactly 

 to this. From experimentation, it appears that this series of nerve 

 endings is specialized for the detection of pressure changes ; thus it is 

 useful to the fish for determining direction of flow and depth of water. 

 In our earth-bound existence we have no need for such a sense. 



In its simplest form, a receptor is simply a free nerve ending, but 

 by development of accessory structures and the grouping together of 

 many nerve endings, complex receptors such as the eye and the ear 

 are developed. In between these two extremes all degrees of develop- 

 ment may be found. 



The sense organs may be divided into two principal groups : those 

 which receive stimuli from the external environment, the exteroceptors; 

 and those which respond to stimuli from the internal environment, that 

 is, from within the body itself, the interoceptors. The exteroceptors 

 may be further divided into those which respond only to direct stimuli 

 and those which can respond to stimuli at a considerable distance from 

 the body. In the first group can be included those receptors for touch 



