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. Hence 

 this 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 since 

 afforded protection to it, while all other organs of its kind have 

 disappeared from the surface of terrestrial animals. The process 

 of 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 the 

 morphological surface of the body, yet they are kept immersed 

 in a watery fluid. 



Cut 26. — This diagram illustrates the relative position of the surface sense organs 

 and 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 less 

 curved structures, because they are formed out of a portion of 

 the external surface of the body, which- has been inclosed within 

 the head. Since on the surface of the body, in the typical and 

 usual process of development, the canals always open at both 

 ends on the surface, so inside the auditory vesicle they open at 

 both ends. Since they are laid down as curved grooves, they 

 naturally hold that form as canals. The mechanical conditions 

 of their development would alone cause them to assume the 

 semicircular or curved shape. It must not be supposed that an 



