GENERAL ORGANOLOGY. 153 



the utriculus with the three semicircular canals. In addi- 

 tion there is formed in the mammals, as also in most verte- 

 brates, a sound-conducting apparatus, so that the auditory 

 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 not formed after the type of 

 auditory vesicles. Still we have no certain knowledge of 

 this except in the case of the tympanal auditory organs of 

 the grasshoppers (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, condition the sensa- 

 tion of equilibrium of the body. For, after these canals are 

 destroyed, the animals begin to stagger and lose their bal- 

 ance. It is possible that in fishes this is the sole function 

 of the labyrinth ; for it has not yet been definitely de- 

 termined that fishes hear. Starting from this assumption, 

 recent investigators have attempted to prove that the 

 auditory vesicles of invertebrated animals are exclusively, or 

 at least preponderatingly, organs of equilibration. 



The Eye is in all animals recognized by the character- 

 istics of the sensory epithelium, the retina. The latter is 

 always marked by strong deposition of pigment which lies 

 either in the sensory cells or in special cells arranged be- 

 tween or behind the sensory cells. The simplest-formed 

 eye, therefore, appears as a pigment-spot in the epithelium 

 of the skin sharply circumscribed, provided with nerves, 

 commonly also with a lens (Fig. 78). 



The Rods and Cones. The sensory cell itself bears 

 usually at its peripheral end a projection, called the rhab- 

 dom. The rhabdom is a sort of cuticular structure, 

 probably serving also to collect the rays of light and to 

 transform them into stimuli, and has, particularly in the 

 vertebrates, a complicated structure, inasmuch as each 

 rhabdom consists of an inner and an outer portion. Here 



