General principles of zoology . d characteristic structure, whichrenders possible a ready recognition, through the almostinvariable presence of an easily recognizable apparatus ac-cessory to the sensory cells themselves, which are essentialfor sensation. History of the Auditory Organs.—The auditory organsof vertebrates and of most of the other animal groups canbe traced back to a simple fundamental form, the auditoryvesicle (Fig. 76). This has an epithelial wall, a fluid con-tents, the endolympJi, and an auditory ossicle or otolit/i,formed from a single or from several fused auditory con-creti
General principles of zoology . d characteristic structure, whichrenders possible a ready recognition, through the almostinvariable presence of an easily recognizable apparatus ac-cessory to the sensory cells themselves, which are essentialfor sensation. History of the Auditory Organs.—The auditory organsof vertebrates and of most of the other animal groups canbe traced back to a simple fundamental form, the auditoryvesicle (Fig. 76). This has an epithelial wall, a fluid con-tents, the endolympJi, and an auditory ossicle or otolit/i,formed from a single or from several fused auditory con-cretions. In a definite region of the epithelial wall thecells are developed into the crista aciistica, the auditoryridge; they are in connection with the auditory nerve(nervus acusticus) and bear the auditory hairs projectinginto the endolymph. The otoliths themselves are concre- 152 GENERAL PRINCIPLES OF ZOOLOGY. tions of carbonate or of phosphate of lime, whose functionis not yet explained. They usually float free in the centre. FIG. 76.—Auditory vesicle of a Mollusk (Pterotrackea). N, auditory nerve ; //=, auditorycells with the central cell; €2, ciliated cells; Mz, ciliated cells; Of, otolith. (AfterClaus.) of the vesicle, and are often held in place by bunches ofcilia which project from the non-sensitive epithelial cells. Auditory Pit.—Every auditory vesicle develops from apitlike invagination of the skin, and consequently is for atime an auditory pit. Therefore it is not surprising that inmany animals the organ has stopped at the lower stage ofdevelopment; for example, the crayfish has an open audi-tory organ or auditory pit. On the other hand, the auditoryvesicle may assume a great variety of forms. In thevertebrates, for example (Fig. 77), it is divided by a con-
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
