194 



FOUNDATIONS OF BIOLOGY 



spending to environmental changes. Thus Paramecium re- 

 acts to mechanical, thermal, chemical, and electrical stimula- 

 tion : the en-tire surface of the cell is sensitive to stimuli, and 



the excitations are conducted from 

 one part to another essentially by 

 the protoplasm as a whole. In some 

 Invertebrates, such as Hydra and 

 the Earthworm, the whole surface 

 of the body is still depended upon 

 as a receiving organ for all kinds of 

 stimuli, and only simple sense 

 cells are developed. In the major- 

 ity of animals, however, although all 

 the cells retain to some extent their 

 pristine power of irritability, envi- 

 ronmental changes exert their influ- 

 ence chiefly upon complex receptors, 

 which are specialized to respond 

 most readily to particular forms of 

 energy. The energy, for example of 

 heat or light, is transformed by ap- 

 propriate mechanisms into the ener- 

 gy of a NERVE IMPULSE, and accord- 

 ingly the sense organs constitute 

 the outposts of the nervous system. 

 Since we necessarily gain our knowledge of the outside world 

 solely through the data afforded by our sense organs, it fol- 

 lows that we judge the capacity of the sense organs of other 

 animals merely by analogy with our own. This is a safe pro- 

 cedure only in the case of sense organs which more or less 

 correspond in structure to those which we possess. In the 

 Crayfish, for example, we find complex sense organs which, 

 without doubt, are eyes, and others which are ears, or at least 



ABC 



FIG. 108. Diagram of stages 

 in the differentiation of sense 

 cells. A, primitive sensory neu- 

 ron of Hydra-like animals; B, sen- 

 sory neuron of a Mollusc; C, 

 primary sensory neuron of a 

 Vertebrate. In each case the 

 sensory surface is represented 

 below, and therefore the nerve 

 impulse passes upward. (After 

 G. H. Parker.) 



