THE SENSE ORGANS 



595 



stimulate the hair cells which are the sensory elements of the organ. 

 Grains of sand occasionally replace the otoliths. 



The diversity of static organs in invertebrates is, however, so great 

 that we must conclude that they have been independently acquired in 

 the different groups. The fact that similar statocysts occur in such 

 diverse forms as echinoderms, annelids, and molluscs points in the same 

 direction. 



Auditory organs have not been demonstrated in aquatic invertebrates. 

 Indeed, it has not been demonstrated that any fishes can hear. Fishes 

 respond to blows upon the surface of the water, but this may involve the 



Fig. 493. — Diagram of mammalian ear. a, ampullae of semicircular canals; an, 

 acoustic nerve; en, cochlear nerve; em, external auditory meatus; eu. Eustachian tube; 

 ft, fenestra tympani; i, incus; m, malleus; p, perilymph space (black); pd, perilymph 

 duct; ph, pharynx; s, stapes; sc, sacculus; sg, spiral ganglion; sm, st, su, scalae media, 

 tympani, et vestibuli; t, tympanic cavity; tm, tympanic membrane; u, utriculus; vn, 

 vestibular nerve. (From Kingsley's "Comparative Anatomy of Vertebrates.") 



lateral line organs or the sense of touch, not the ear. Auditory organs 

 may be unnecessary among animals which are themselves unable to 

 produce sounds. 



Among invertebrates, true auditory structures are represented by the 

 chordotonal and tympanic organs of insects. But neither these nor any 

 other organs of invertebrates have a. genetic relation to the ears of 

 vertebrates. 



Static organs occur in the free-swimming urochordates. In the larvae 

 of ascidia and phallusia, a static organ with ciliated sensory epithelium, 

 statolith, and nervous connexions projects into the brain cavity. Nothing 

 similar is found in vertebrates. Amphioxus lacks a static organ, and is 



