594 



COMPARATIVE ANATOMY 



STATIC AND AUDITORY ORGANS 



A wide-spread and remarkable trait of animals is the ability to respond 

 to gravitation, and thus to orient their bodies in space. The long axis 

 of the body is usually kept horizontal, but occasionally, as in man, vertical. 



With few exceptions, animals have 

 dorsal and ventral sides. But the 

 ventral side does not lie nearer the 

 earth because it is heavier, for when 

 a fish dies in the water it turns 

 ventral side up. Orientation in 

 relation to gravity is a reflex act, 

 which involves nervous and mus- 

 cular mechanisms and special sense 

 organs. Such organs are known 

 as static organs. 



Some microscopic floating 

 plankton organisms are indifferent 

 to gravity. When these maintain 

 a constant position the stable 

 balance is due to the greater weight 

 of one side. Where special static 

 organs are lacking, orientation may 

 be maintained through eyes or 

 ^. ^TTT^ u tactile organs or both. Some 



Fig. 492. — Diagram of the membranous o . . 



labyrinth of a vertebrate, the sensory areas crustacea, for example, Will SWim 



dotted, ac, anterior semicircular canal; j^g ^^^^ when the body is 



ap, ampullae; ca, cristae acusticae m the t^ a i 1, u 



ampullae; de, ductus endolymphaticus; he, illuminated from below. Although 



horizontal (external) canal; /. lagena; -^ organs are generally absent 



ml, mn, ms, mu, maculae (of lagena, neglecta, o _ » i j 



sacculi and utriculi) ; pc, posterior semi- in insectS, mOSt inseCtS, placed On 

 circular canal; s, sacculus; se, saccus 

 endolymphaticus; sue, sacculo-utricular 



canal; u, utriculus 



"Comparative Anatomy of Vertebrates 



the back, will right themselves, 

 (From Kingsley's probably through the action of 

 ^ nerves connected with tactile hairs. 

 The normal position of many animals such as worms brings the ventral 

 side of the body in contact with the bottom. When this contact is lost, 

 reactions tending to restore it take place. The sensory basis of this 

 orientation is the sense of touch. In the orientation of the mammalian 

 body, tactile organs, muscular spindles, and the semicircular canals of the 

 ear are all involved. 



Among invertebrates most static organs have a hollow sac or statocyst 

 which contains one or more statoliths, which are granules of calcium 

 carbonate or sulphate mixed with organic matter. Frequently, loose 

 crystals or otoconia occur in statocysts, and serve by their motion to 



