188 ESSENTIALS OF ZOOLOGY 



which conducts the vibrations finally to the organ of Corti. The latter 

 is supported by a membrane dividing the cochlea throughout its 

 length. Endings of fibers from the eighth (auditory) cranial nerve 

 reach the cells in this organ and are stimulated by the vibrations of 

 the fluid. The average human ear is said to receive wave lengths of 

 frequencies ranging between 30 and 40,000 per second, but other 

 animals including the rat likely perceive wave lengths of a different 

 range. 



Sense of Equilibrium. — The three semicircular canals which were 

 mentioned above stand in three different planes and all join the 

 vestibule or general cavity at their bases. One is anterior- vertical in 

 position, another posterior-vertical or at right angles to the first, and 

 the third is in the lateral horizontal position. There are sensory 

 endings of branches of the auditory nerve in the ampullae of these 

 canals, and the canals are filled with a lymphlike fluid. Any change 

 of level or position of the head causes this fluid to move in the canals 

 and stimulate certain of the endings. The individual is able to sense 

 the position and orientation of the body from the impulses produced 

 by these stimulations. 



Excretion 



A certain result of the oxidation necessary for metabolism is the 

 production of end-products (principally nitrogenous) which are not 

 only of no further use to the protoplasm but may be a distinct 

 menace to the welfare of the organism because of their toxic effects. 

 The substances are usually dissolved and removed as a waste liquid 

 or occasionally as crystals by special parts of the body. 



In Protozoa this function is performed by general diffusion 

 through the plasma membrane and in many forms by the contrac- 

 tile vacuoles. The quantity of water which passes through the pro- 

 tozoan in tw^enty-four hours is several times the volume of the 

 animal itself. Among sponges and coelenterates diffusion of liquid 

 wastes through the general surfaces of the body to the surrounding 

 water serves for excretion. 



In an animal like the flatworm, Planaria, excretion is accomplished 

 by a system of canals which begins in numerous capillary-sized tubules 

 whose blind ends are composed of individual cells called flame cells. 

 These flame cells are irregular in shape and each bears a tuft of cilia 

 extending into the end of the tubule. The flickering movement of the 

 cilia in the cell gives the appearance of a flame and moves the accumu- 



