434 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



When the outer surface of the body becomes 

 hardened by the dejposit in its wall of chitin, as in the 

 Arthropoda, or of calcareous salts, as in the Echino- 

 dermata, or by the development of a shell, as in the 

 Mollusca, the general tactile sense becomes more dis- 

 tinctly limited ; this is, perhaps, least noticeable in the 

 Ediinodcrmata, where the superficial plexus of 

 nerve filaments extends over the test and along the 

 projecting suckers, while special nerve cells are deve- 

 loped in the highly sensitive pedicellariae. 



In the Arthropoda the special tactile organs are 

 seen at their simplest in Peripatus ; in it the dorsal 

 surface is raised up into delicate imbricated papillae, 

 from the tip of which there projects a fine process. In 

 others they take on the forms of projecting rods. As 

 we all know, we have only to stroke lightly the hairs 

 on our own arms to discover how easily tactile sensa- 

 tions are conveyed by more or less stiff processes to 

 the sensory cells that lie at their base. Where the 

 greater part of the integument is hardened it is clear 

 that projecting rods or "hairs" will, if they be 

 provided with nerve fibres, and continuous with 

 sensory cells, convey to the underlying and protected 

 nervous system any movement of their free ends ; the 

 movement, then, of these hairs becomes in an Arthro- 

 pod a sense of touch ; these rods are not confined to 

 the antennae, for they are developed on very various 

 parts of the bodies of Arthropods. 



Sagitta, in which there is likewise a chitinous 

 cuticle investing the body, has a large number of 

 bundles of stiff setae scattered over the surface of its 

 integument (Fig. 186). 



Among the Chordata we find that little is 

 definitely known as to the tactile organs of the two 

 lower groups ; the only sensory cells that have as yet 

 been recognised in Amphioxus are of a much sim- 

 pler character than those which we have just been 



