. A manual of zoology. Zoology. [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 li

. A manual of zoology. Zoology. [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 li Stock Photo
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. A manual of zoology. Zoology. [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

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