582 VBRTEBHATE LIFE AND ORGANIZATION 



hair cells. Sudden turns of the head in various planes (angular accelera- 

 tion) induce movements ol the endolymph within the semicircular 

 canals, which in turn stimulate hair cells in the ampullae. 



Phonoreception in Fishes. The part of the ear concerned with 

 equilibrium is essentially the same in all vertebrates, but the part con- 

 cerned with phonoreception or hearing, that is, the detection of sound 

 vibrations, differs considerably among vertebrates. Mammals, birds and 

 some reptiles have a cochlear duct, an elongated cul-de-sac extending 

 from the sacculus which is clearly concerned with phonoreception. 

 Fishes have a homologous but very small diverticulum known as the 

 lagena. The rudimentary nature of this structure, together with early 

 experiments in which fishes were shown to be unresponsive to sounds 

 made in the air, led to the conclusion that they could not hear. Later 

 this conclusion was questioned when it was realized that most air-borne 

 sound waves are reflected by the air-water surface, and when it was dis- 

 covered that there are a great many sounds produced in the water by 

 aquatic organisms. Dr. Moulton of Bowdoin College has been able to 

 induce, or to suppress, the staccato calls of the sea robin (Prionotus) by 

 appropriate underwater noises! Sound waves travel rapidly in the water 

 and pass without interruption through the flesh of a fish; tissues have a 

 high content of water. More recent experiments by Dr. von Frisch of 

 the University of Munich and Dr. Griffin of Harvard University have 

 shown that many fishes respond to underwater sounds of a wide range of 

 frequencies provided the sacculus and lagena are intact. Catfishes and 

 some other fishes that are particularly sensitive to sounds apparently 

 use their swim bladder as a hydrophone. This picks up vibrations pass- 

 ing through a large part of the body, and transmits them via a chain 

 of small bones derived from the vertebrae (Weberian ossicles) to the 

 sacculus and lagena. 



Clearly, fishes can detect underwater sounds by means of a part of 

 the membranous labyrinth. In addition, fishes have a lateral line system 

 that is sensitive to currents, to changes in pressure and to vibrations of 

 low frequency. It consists of a longitudinal canal extending the length 

 of the trunk and tail, and of a series of canals that ramify over the head. 

 These canals are embedded in the skin and connect with the surface 

 through pores. Water enters these canals and stimulates hair cells in the 

 lining similar to those in the ear. Neurons from these receptors enter 

 an acoustico-lateralis area of the brain along with neurons from the 

 ear, which suggests that there is a close relationship between the ear 

 and lateral line. The inner ear develops embryonically in close asso- 

 ciation with certain lateral line canals, and it may have evolved in the 

 same way. Larval amphibians have a lateral line system, but it is lost 

 during metamorphosis. Higher vertebrates never have this system at all. 



Phonorecepffon in Tetrapods. In all tetrapods, a part of the mem- 

 branous labyrinth, generally the lagena or cochlear duct, is specialized 

 for phonoreception, and various devices have evolved which transmit 

 either ground or air-borne vibrations to it. Frogs have an external tym- 

 panic membrane (Fig. 21.17) which responds to vibrations in the air, 

 and a stapes, which transmits the vibrations across the middle ear 



