394 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



undergo aggregation, so that a diffused or scattered 

 arrangement makes way for one in which we have 

 definite nerve centres and well-marked lines, along 

 which, and along which only, nervous impulses will 

 pass. The most important of the aggregations of 

 nerve cells form the brain, or cerebrum, the most 

 important of the fibres the nerve cords; and just 

 as nerve fibres going to or coming from the latter are 

 associated with them, so there are secondary masses 

 of ganglia which are connected with the former. 



In the next place, information from without is 

 gained from specially-modified cells, sense cells ; 

 these belong to the epithelial region of the body, and 

 are derivates of the epiblast. 



The most generalised and widely-distributed sense 

 cells are those which belong to the sense of touch, the 

 so-called tactile cells; next we have those which, 

 only a little more complex, are confined to the ante- 

 rior region of the digestive tract ; these are the 

 gustatory cells, or those that subserve the sense of 

 taste ; thirdly, we have the more complicated organs 

 of the three higher senses, smell, sight, and 

 hearing^. When a brain is developed, all, or such 

 of these organs as are present, send to it by the 

 nerve fibres messages from the outer world ; in it 

 the messages are converted into more or less distinct 

 sensations, and from it fresh messages are sent out to 

 the different parts of the body. 



The relations of the sensory cells to the epithelial 

 layer are particularly well seen in some of the 

 Coelenterata ; for example, in the sea-anemones 

 (Tealia; Fig. 168), some of the cells of the epithelial 

 layer have their free end continued into a fine 

 stiff" process, which projects outwards ; the inner or 

 basal end of such cells breaks up into finer pro- 

 cesses, which branch towards their ends. The free 

 projecting process may be justly regarded as a 



