MULTICELLULAR ANIMALS 



229 



move synchronously in the socket and that both eyes are directed 

 at the object of vision. If there is an unbalance between the mem- 

 bers of any pair of these muscles, that eye will be out of symmetry 

 with its fellow, either a crossed or a wall-eye. Correction may be 

 made by the use of lenses, or by operation to correct the difference 

 in tension between the muscles. 



The Ear of Vertebrates. The sense of hearing is essentially 

 a sensitivity to vibrations of certain frequencies. Many of the simple 

 animals are sensitive to sound frequencies, but whether their re- 



EDG€ OF ^v.^^y- 

 UMBRELLA -^^ry^h 



SENSE ORGAN 



SAC 



CONCRETIONS 



Fig. 157. — Statocysts, the balancing organs in a jelly fish. (After Parker and Haswell: 

 Textbook, of Zoology, published by The Macmillan Company.) 



sponse is due to reception of mechanical vibrations or to sounds as 

 distinct from mechanical effects is difficult to determine. Special 

 organs for sound reception find their highest development among 

 the higher vertebrates. The essentials of the vertebrate ear are: A 

 membrane under such tension that it vibrates with the vibrations of 

 the air or water of the environment; a mechanism consisting of a 

 bone or a series of bones which transfers these vibrations to an 

 organ that analyzes the vibrations so that they activate the appro- 

 priate nerve ending. The organ of hearing is associated with an 

 organ which detects changes in position by the dislocation of fluids 

 and small concretions; when the animal changes position these 

 stimulate appropriate nerve endings. Similar organs that function 

 in maintaining position and equilibrium occur in invertebrates 

 (Figs. 157 and 158). 



