Journal of morphology . n about what was going onoutside, at the same time permitting the apparatus to occupythe least possible space. The ear Anlage was so placed that itmight be housed inside the head, leaving the surface of thebody free for sense organs of another degree of specialization. 222 AYERS. [Vol. VI. No other area of equal size on any other part of the body-could give the animal such good physiological service. Hencethis area with its sense organs was specially suited to be differ-entiated by natural selection, and its early and sufficient invo-lution and burial within the tissues
Journal of morphology . n about what was going onoutside, at the same time permitting the apparatus to occupythe least possible space. The ear Anlage was so placed that itmight be housed inside the head, leaving the surface of thebody free for sense organs of another degree of specialization. 222 AYERS. [Vol. VI. No other area of equal size on any other part of the body-could give the animal such good physiological service. Hencethis area with its sense organs was specially suited to be differ-entiated by natural selection, and its early and sufficient invo-lution and burial within the tissues of the head have ever sinceafforded protection to it, while all other organs of its kind havedisappeared from the surface of terrestrial animals. The processof involution has enabled the animal to preserve primitive con-ditions— the necessary conditions of existence for such hair-bearing structures. Although the ear sense organs lie on themorphological surface of the body, yet they are kept immersedin a watery Cut 26. — This diagram illustrates the relative position of the surface sense organsand their canals with their nerve connections in the Ganoid Dogfish. After Allis(4), PI. XLII, Fig. 49. The so-called semicircular canals develop as more or lesscurved structures, because they are formed out of a portion ofthe external surface of the body, which- has been inclosed withinthe head. Since on the surface of the body, in the typical andusual process of development, the canals always open at bothends on the surface, so inside the auditory vesicle they open atboth ends. Since they are laid down as curved grooves, theynaturally hold that form as canals. The mechanical conditionsof their development would alone cause them to assume thesemicircular or curved shape. It must not be supposed that an No. I.] THE VERTEBRATE EAR. 223 exact semicircular shape is the usual form of these canals, forsuch is far from being the case, and I have very great doubtsthat the mathematica
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