128 GENERAL PRINCIPLES OF ZOOLOGY. 



Auditory Pit. — Every auditory vesicle develops from a i^itlike 

 invagination of the skin, and consequently is for a time an auditory 

 pit. Therefore it is not surprising that in many animals the organ 

 has stopped at the lower stage of development; for example, the 

 crayfish has an open auditory pit. On the other hand, the 

 auditory vesicle may develop a complicated system of cavities as 

 in mammals (fig. 80), where it is divided by a constriction into 



-V 



Fig. 80— Diagram of the human labyrinth. ('. iitriculus with the semicircular 

 canals: .*^, sacculus connected witl:i the cochlea (C) by the canalis reuniens; R, 

 recessus labyrinthi; 1^, blind sac of the cochlea; K, apex of the cochlea. 



the sacculus and the utriculus. The sacculus is jDrovided with a 

 spirally-wound blind sac, the cochlea, the utriculus with the three 

 semicircular canals. In addition there is formed in the mammals, 

 as also in most vertebrates, a sound-conducting apiparatus, so that 

 the auditor)' organ acquires an extremely complicated structure. 



Other Forms of Auditory Organs. — Since there are animals 

 without auditory vesicles which hear well, like the spiders and 

 insects, we must assume that there are auditory organs which are 

 formed after another type. Still we have no certain knowledge of 

 these except in the case of the tympanal auditory organs of the 

 grassho^jpers (which compare). 



Function of the Semicircular Canals. — Experiments upon 

 representatives of the most diverse classes of vertebrates have led 

 to the conclusion that the three semicircular canals, standing at 

 right angles to each other, are organs of equilibrium, for, after 

 these canals are destroyed, the animals begin to stagger and lose 

 their l)alance. It is probable that in fishes this is the sole function 

 of the labyrinth; for it has not been determined that fishes hear. 

 Starting from this assumption, recent investigators have attemjited 

 to prove that the auditory vesicles of invertebrated animals are 

 exclusively, or at least largely, organs of equilibration. This would 

 explain the otoliths, for these budics. of relatively large specific 

 gravity, would affect the crista in different ways according to the 

 position of equilibrium of the body. Statoliths would thus be a 

 better name. 



