MORPHOLOGY OF ANIMALS 



tympanal organ may be present, consisting of a vibrating 

 membrane overlying a trachael chamber (Fig. 15, A, /) ; 

 sense cells are present between the membrane and chamber, 

 and when the membrane is set into vibration by sound waves 

 the sense cells are stimulated. 



Lateral Line Organs. Among aquatic vertebrates (fishes 

 and amphibians) a system of integumentary sense buds is 

 found along the right and left sides of the body and over the 

 head, which is known as the lateral-line system. The func- 

 tion of these organs is to receive vibrations of low frequency 

 such as waves in water; probably they are organs also of 

 equilibration. In all vertebrates it is probable that the 

 auditory organs, as well as the organs of smell and taste, 

 have been derived from integumentary sense organs homolo- 

 gous with those of the lateral line. 



The Vertebrate Ear. In the process of development the 

 ear of vertebrates appears as a pit-like invagination of the 

 skin which is then infolded to form a vesicle; this vesicle 

 then becomes partly divided into two chambers, the utricle 

 and the saccule. In most vertebrates the former bears three 

 pairs of semi-circular canals which are organs of equilibra- 

 tion, while the latter gives rise to a recess, the lagena, or 

 cochlear duct, which is the specific auditory organ. Cal- 

 careous concretions or otoliths are frequently present in the 

 utricle (Fig. 27, A). These sensory portions of the auditory 

 organ are known as the inner ear; to this is added in all 

 animals above the frogs and toads a middle ear or tympanum 

 which develops out of the first gill cleft and which transmits 

 the sound waves from the surface to the inner ear. Finally, 

 in mammals there are folds of the integument around the 



C 79 3 



