GENERAL ORGANOLOGY. 



cells, the olfactory cells. In the air-breathing vertebrates 

 the cavity becomes part of the canal which serves for respira- 

 tion, leading from the surface of the skin into the pharynx. 

 Consequently we shall be inclined to regard as organs of 

 smell sensory organs of invertebrates, (e.g., medusa, 

 ccplialopods), which have the form of ciliated pits and lie near 

 the respiratory apparatus (e.g. the osphradeum of molluscs). 

 Yet there are exceptions. Experiments seem to show that 

 in the arthropods the antennae probably serve for smelling. 

 Here the sensory perception can be connected only with 

 certain modified hairs, the olfactory tubules of the Crustacea 

 and the olfactory cones of insects. In an analogous way 

 we shall term as organs of taste certain nerve-endings 

 directly within or in the neighborhood of the pharynx, 

 since the taste-organs of vertebrates, the so-called taste-buds, 

 have been observed in the mouth-cavity, especially on the 

 tongue. 



Organs of Hearing and of Sight are called the higher 

 sense-organs, because they are of much greater importance 

 in the totality of our knowledge than the other organs, 

 since they furnish sensations which are quantitatively and 

 qualitatively much more definite. Ears and eyes have 

 therefore a complicated and characteristic structure, which 

 renders possible a ready recognition, through the almost 

 invariable presence of an easily recognizable apparatus ac- 

 cessory to the sensory cells themselves, which are essential 

 for sensation. 



History of the Auditory Organs. The auditory organs 

 of vertebrates and of most of the other animal groups can 

 be traced back to a simple fundamental form, the auditory 

 vesicle (Fig. 76). This has an epithelial wall, a fluid con- 

 tents, the endolympJi, and an auditory ossicle or otolit/i, 

 formed from a single or from several fused auditory con- 

 cretions. In a definite region of the epithelial wall the 

 cells are developed into the crista aciistica, the auditory 

 ridge ; they are in connection with the auditory nerve 

 (nervus acusticus) and bear the auditory hairs projecting 

 into the endolymph. The otoliths themselves are concre- 



