THE SENSE ORGANS 



415 



Unpaired Eyes 



Two kinds of median eyes are recognized, the parietal and the pineal. 

 Each, when present, arises from the roof of the diencephalon, and lies 

 beneath an unpigmented "apical spot" on the upper surface of the head. 



That parietal and pineal eyes are light-recipient organs is indicated by 

 the presence of a retina with photoreceptors, pigment, ganglion cells, 

 nervous connexions with the brain, 

 and sometimes a lens. Experi- 

 mental evidence is somewhat con- 

 flicting. But lizards show a 

 muscular response to photic stimu- 

 lation of the parietal organ, and 

 teleosts react to stimulation of the 

 pineal organ. 



The history of the median eyes 

 of vertebrates is one of reduction 

 and of functional change. Among 

 the possible factors in the degener- 

 ation of these organs may be the 

 increased importance of the lateral 

 eyes and the great enlargement of 

 the cerebral hemispheres, which in 

 mammals overlie the diencephalon. 



STATIC AND AUDITORY ORGANS 



A wide-spread and remarkable -p,^ ,^. t-.- t .u u 



_ _ _ J:"iG. 300. — Uiagram of the membranous 



trait of animals is the ability to labyrinth of a vertebrate, the sensory areas 



respond to gravitation, and thus to ^°"''^- "'• ^"'^"°' semicircular canal; 



/ _ ^ _ _ ' ap, ampullae; ca, cnstae acusticae in the 



orient their bodies in space. The ampullae; de, ductus endolymphaticus; he, 



long axis of the body is usually kept 1^°"'°"'^^ (external) canal; ;. lagena; ml. 

 . . ; ' ' ' inaculae (of lagena, neglecta, 

 horizontal, but occasionally, as in sacculi and utriculi); pc, posterior semi- 

 man, vertical. With few excep- T'^^lf^^^^^'^ ^' '^^^^^'= ""• 'T''' ''"'^T 



'^ lymphaticus; sue, sacculo-utricular canal; 



tions, animals have dorsal and «. utricuius. (From Kingsiey's "Com- 



Ventral sides. But the ventral P^rative Anatomy of Vertebrates.") 



side does not lie nearer the earth because it is heavier, for when a 

 fish dies in the water in turns ventral side up. Orientation in relation to 

 gravity is a reflex act, which involves nervous and muscular mechanisms 

 and special sense organs. Such organs are known as static organs. 



Some microscopic floating plankton organisms are indififerent 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 tactile organs or 



