[16 



GENERAL TRENXIPLES OF ZOOLOGY 



aliove or in front of the mouth. In Ihc air-breathing vertcliratcs this 

 pair of pits, which here also arise from the skin, are taken into the dorsal 

 wall of the two respiratory canals leading from the outside to the mouth 

 or pharynx. Now since the olfactory cells distributed in these pits (hg 

 38. B) are frequently characterized l)y bundles of olfactory hairs, while 

 the surrounding epithelium is often ciliated, one is inclined to regard as 

 organs of smell sensory organs of invertebrates, which have the form of 

 ciliated pits or lie near the respiratory apparatus (e.g., the ospliradiiim of 

 molluscs). Yet there are exceptions. Experiments ceem to show that in 

 the arthropods the antenna? serve for smelling. Here the sensory per- 

 ception can he connected onlv with certain modified hairs, the olfactory 

 tubules of the Crustacea and the olfactory cones of insects. In a similar 

 way certain nerve end organs in the region of the mouth are considered 

 as organs of taste, since the taste organs of vertebrates, the so-called taste 

 buds, are aliundant in (he mouth cavity. 



Organs of Hearing and of Sight are called the higher sense-organs, 

 because they are of much greater importance than the other organs, 



Fig. 82 — ,-\uditory vesicle of a mollusc (Plcrdrarhci). .\', aiulitory nerve; Hz. audi- 

 tory cells with Ine cenlral cull; Cz, \Vz, cllialed tells, O.', otolilii' (.after Claus). 



since they furnish sensations which are quantitatively and qualitatively 

 much more definite. Ears and eyes have therefore a complicated and 

 chr.racteristic structure, which renders them easily recognizable by the 

 almost invarialjle presence of certain structures accessory to their 

 functions. 



The auditory organs of vertebrates and of most other animal groups 

 can be traced Ijack to a simple fundamental form, the cuidilorv vesicle 

 (fig. 82). This has an epithelial wall, a fluid contents, the endolymph, 

 and an auditory ossicle or otolith, formed from one or from several 

 fused concretions. In some instances the otoliths, to the number of 



