222 THE POPULAR SCIENCE MONTHLY. 



lows by means of connecting fibres, while they are also more distantly 

 united with other nerve-ganglia in different parts of the body by means 

 of commissural fibres. 



But another special sense-endowment remains to be referred to. 

 This has to do with the organism's power of appreciating sounds or 

 " auditory " expressions a power which is, however, probably pos- 

 sessed in only a low degree by most invertebrate animals ; since, even 

 in the most perfect form of the organ of hearing among them we 

 have to do with a very rudimentary structure. In this respect there 

 is a great difference between the sense of sight and the sense of hear- 

 ing. While the eye of the cuttle-fish attains a degree of elaboration 

 that does not fall so very far short of the most perfect form which it 

 displays among vertebrate animals, the organ of hearing, as a mere 

 organ, in all forms of the invertebrata is remarkable for its simplicity, 

 and remains notably inferior to the highest type attained by this sen- 

 sorial apparatus which, with its nerve-connections, becomes so enor- 

 mously developed in many mammals and in man. 



Like the sense of sight and the sense of smell, that of hearing, even 

 in its simplest grades, serves to bring the organism into relation with 

 more or less distant bodies, so long as they are sufficiently sonorous 

 to transmit the so-called " sound " vibrations through water or air to 

 the sensitive organs which become attuned to receive such impres- 

 sions. 



An auditory organ does not seem to be present at all certainly 

 none has as yet been detected or inferred to exist in many of the low- 

 er forms of life ; while in other animals, though inferred to exist, it 

 remains as yet unrecognized. This is the case, for instance, with the 

 majority of Crustacea, spiders, and insects. Judging from the instances 

 in which an organ of hearing has been detected in mollusks, and in a 

 very few representatives of the classes above named, it seems (however 

 novel the information may be to many readers) that it is an organ of 

 special sense which is not habitually, or even usually, found in the 

 head, and in direct relation with one of the ganglia composing the 

 brain. Further remarks, however, on this subject must be deferred 

 until a brief description has been given of the nature and distribution 

 of the nervous system in some of the principal groups of invertebrate 

 animals. 



These, then, are the commonly-received modes by which organisms 

 are impressed from without, and by which they attune themselves to 

 the conditions and actions in their medium. It was recognized by 

 Democritus, and other ancient writers, that they are all of them deriv- 

 atives, or more specialized modes of a primordial common sensibility, 

 such as is possessed by the entire outer surface of the organism. Touch, 

 taste, smell, vision, and hearing, are sense-endowments, having their 

 origin in organs formed by a gradual differentiation of certain portions 

 of the external or surface layer of the body that is, of the part in 



