4i6 CHORDATE ANATOMY 



both. Some Crustacea, for example, will swim upside down when the 

 body is illuminated from below. 



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 o.rientation of the mammalian 

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

 ear are all involved. 



Among invertebrates most static organs consist of a hollow sac or 

 statocyst containing one or more statoliths, which are granules of cal- 

 cium carbonate or sulphate mixed with organic matter. Frequently, loose 

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

 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 several groups. The fact that similar statocysts occur in such 

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

 direction. 



Auditory organs have not been demonstrated in aquatic invertebrates. 

 Indeed, it has not been demonstrated beyond question that any fishes 

 can hear. Fishes respond to blows upon the surface of the water, but 

 this may involve the lateral-hne organs or the sense of touch, not the ear. 

 Auditory organs may be unnecessary among animals which are them- 

 selves 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 

 quite unable to maintain its balance in swimming. 



All vertebrates have a static organ, which is a novelty in this group 

 and not a structure inherited from invertebrate ancestors. In fishes and 

 aquatic urodeles, the membranous labyrinth of the ear appears to be 

 exclusively a static organ. But in land forms, beginning with the 

 amphibians, the ear has the double function of equiUbration and hearing. 



Of vertebrates, the cyclostome Myxine has the simplest static organ, 

 which in shape is not unlike an inflated inner tube of an automobile tire 

 with some inequalities of expansion. From its nerve supply, it is thought 

 to correspond to the utriculus and the two vertical semicircular canals 



