20 



General Part. 



Fig. 19. Auditory capsule of a Gastropod, 

 Ot otolith, N auditory nerre. — After Clans. 



either remain as sacs, opening freely on the surface : or more often 

 are entirely cut ofE from the epidermis, and are therefore closed 



sacs and frequently lie deep 

 in the body. The auditory 

 capsules are usually almost 

 spherical, but in the Verte- 

 brata they are more compli- 

 cated, as will be seen later. 

 The wall into which the nerve 

 fibres (auditory nerve) pass, 

 consists of a simple epi- 

 thelium, whose cells — all, or 

 only some — are provided with 

 fine hair-like processes pro- 

 jecting into the liquid. These 

 hairs are set in vibration by 

 sound waves, and the sensations thus caused are conducted by the 

 auditory nerve to the central nervous system. One or more hard 

 calcareous corpuscles, the otoliths, are usually suspended in 

 the liquid. Amongst the higher Vertebrates, various kinds of 

 accessory apparatus function as resonators, etc. 



The auditory organ of Arthropods differs essentially from the usual type. 

 In Ci-ustaoea this sense is closely connected with specially developed hairs, which 

 are free on the surface of the body, or contained in cup-shaped invaginations. In 

 Insects again, there is quite a different an-angement, as will be seen in the Special 

 Part. 



It has been proved, experimentally, that the auditory capsules of many 

 animals have yet another function than that of bringing about impressions 

 of sound. They (or in Vertebrata, their semicircular canals) are of significance in 

 the maintenance of the normal equilibrium in an animal. If they are destroyed, 

 movement becomes unsteady, the animal falls on one side, etc.* In what 

 manner, however, the auditory organs exercise this control is not yet understood. 

 Optic Organs of most animals are, like the preceding sense 

 organs, specially developed parts of the epithelium. In its simplest 

 form (Fig. 20, i,) the organ of sight is a little pigmented patch of the 

 epidermis, innervated by a nerve, the optic nerve (some Medusae, a 

 few Lamellibranchs) . In other cases (Fig. 20, 2) the pigmented 

 epidermis forms a small open pit (some Gastropoda, Ccelentera) ; this 

 arrangement may be complicated by having the cuticle, which covers 

 the skin in many lower animals, thickened over the cells forming the 

 depression, so that a lens, a refractile body, is formed (Fig. 20, 3, 

 certain Ccelentera) . Or the cavity is deepened to form a sac with a 

 small external opening, which may contain a gelatinous secretion of 

 the epithelium (Fig. 20, 4, some Gastropods) . Again, by the contrac- 

 tion of the aperture, and by the loss of all connection with the skin 



* In some animals, these occurrences only take place when eyes as well as ears are 

 destroyed, whilst the destruction of eyes alone, or of otooyst alone, has no effect. 



