THE HIGHER ORGANISMS 



165 



ing one or more small crystals or concretions. These 

 vesicles are known as otocysts, the crystals as otoliths. 

 Similar organs, by which vibrations are caught and trans- 

 mitted to the central nervous system, occur among 

 worms and mollusks. They are usually minute, difficult 

 to find, and may be situated in unexpected places, as, 

 for example, in the clam, where they occur in the so- 

 called "foot. " In the crXistacea they are extremely 

 peculiar and consist of small sacs formed by an invagi- 

 nation of the integument, filled with fluid, provided with 



PIQ. 64. Transverse vertical section of Corti's organ of a man twenty-nine 

 years old. es, Limbus laminae spiralis; me, membrana tectoria; Hb, Hensen's 

 striae; mf, fibres of attachment of the membrana tectoria to the zona tecta; at, 

 sulcus spiralis; siz, epithelium of the sulcus spiralis; is, inner supporting cells; 

 ic, inner rod cells in connection with the outer rod cells, between which is seen 

 the tunnel (t) of Corti; ih, inner hair cell; dh^dh*, outer hair cells; dz, Deiters* 

 cells; as, Hensen's supporting cells; rb, nerve fibres of the ramulus basilaris; 

 n^-n 6 , outer bundles of the spiral nerve fibres; rf, radiating tunnel fibres; at 

 inner part of Nuel's space; mb, upper layer of the membrana basilaris; mb', 

 lower layer of the membrana basilaris; tb, layer covering the tympanic surface 

 of the membrana basilaris; Us, ligamentum spirale. (Gruber, after Retzius.) 



many small hair-like projections. Grains of sand entering 

 from the exterior seem to be essential to the perfection 

 of the apparatus, though sometimes concretions of lime 

 salts form in the organ and constitute an improvement. 

 An advance in the development of the apparatus is 

 found among the insects whose "ears" or chordo- 

 tonal organs are provided with a tympanic membrane. 

 These organs are large, and may be situated upon the 

 sides of the abdomen or upon the anterior tibiae. Some- 

 times, however, no auditory organs can be found when 



