66 THE USE AND NATURE 



Judging from the instances in which * auditory sac- 

 cules ' have been detected in Mollusks, and in some few 

 representatives of the classes above named, it seems (and 

 the information may be novel to many readers) that the 

 endowment in question is not habitually, or even usually, 

 found in the head, or in direct relation with one of the 

 ganglia composing the brain of Invertebrates. In some 

 Heteropida, and their allies, however, the ' saccules,' what- 

 ever may be the function to which they are subservient, 

 seem to be in immediate relation with the brain ganglia.* 

 Further remarks on this subject must, however, be de- 

 ferred until a brief description has been given in future 

 chapters, of the nature and distribution of the nervous 

 system in some of the principal groups of the Invertebrata. 



The foregoing are the commonly received modes by which 

 organisms are impressed from without, and by which they 

 attune themselves to the conditions and actions occurring 

 in their medium. It was recognized by Democritus and 

 other ancient writers, that they are all of them derivatives, 

 or more specialized modes of a primordial common sensi- 

 bility, such as is possessed by the entire outer surface of 

 the organism. Touch, taste, smell, vision, and probably 

 hearing, are sense endowments, having their origin in 

 organs formed by a gradual differentiation of certain por- 

 tions of the external or surface layer of the body — that is, 

 of the part in which common sensibility is most frequently 

 called into play. And just as this common sensibility is a 

 crude or general sense of touch, so are the several specinl 

 senses to be regarded as more or less highly refmed modes 

 of the same sense endowment. 



The distribution and arrangement of nerves in the 

 various impressible surfaces have certain characteristics 



* Siebold, "Manuel d'Anat. Comp./' p. 309, Note 1. 



