3 o NERVOUS SYSTEM AND SENSE-ORGANS 



in-pushing from the exterior the cells in question are of ecto- 

 dermic nature. The largest amount of specialization takes place 

 in Mammals, where the " taste-buds ", as the group of cells are 

 called, are associated with small projections or papillae of the 

 surface of the tongue (fig. 1036). 



SMELL 



Many of the lower animals can undoubtedly smeli as well 

 as taste, though to definitely associate this sense with special 

 cells or groups of cells is not at present possible. Our know- 

 ledge is more complete in the case of Arthropods, Molluscs, 

 and Vertebrates, where experiments lead to results of more de- 



Fig. 1037. Olfactory Organs. A, A small part of the outer branch of antennule of a Crayfish is shown at a, with 

 groups of olfactory setae on its under side, enlarged; b, an olfactory seta, further enlarged. B, Tip of feeler of a Milli- 

 pede, greatly enlarged, showing olfactory cylinders among the ordinary tactile bristles, c, Two olfactory cones from 

 feeler of a Wasp, in section, greatly enlarged. 



finite kind. In all cases the stimulus is of a gaseous nature, 

 and in aquatic animals the gases that are smelt are dissolved 

 in the surrounding water. The sense of smell is obviously of 

 great importance as regards adjustment to the environment. By 

 its means food is in many cases detected, while it often enables 

 animals to recognize friends or foes, even when these are at a 

 considerable distance. This is, of course, due to the nature of 

 the stimulus. Since Smell, Hearing, Sight, and the Temperature 

 Sense are able to give information about objects which are more 

 or less far away, they may be grouped together as Distance- Senses 

 (telaesthetic senses), and are in marked contrast to Touch (so far 

 as haptic sensations are concerned), which only conveys knowledge 

 regarding things that actually come into contact with the skin. 



