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 possessed 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 hearing. 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 sensorial apparatus—which, with its nerve-connections, becomes so enormously 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 impressions.
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 lower 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 derivatives, 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
