310 



STRUCTURE OF THE VERTEBRATES 



sensory processes, and the movement of fluid within stimulates 

 the cells and indicates body position. The canals are paired in 

 the lamprey and its relatives. 



The definitive inner ear of the vertebrates is established in 

 the elasmobranchs. To the two dorso-ventral canals a third 

 horizontal one has been added, all of which empty into an en- 



FiG. 167. Development of the Semicircular Canals (Mammal). (A) Shows 



the utriciilus and endolymphatic duct; in (B) the first (transverse) canal 



has developed and the cochlea is elongating; (C) and (D) show further 



progress in development. 



larged central cavity, the utricidus. From the ventral side of 

 the utriculus develops a rounded sacculus which is of great 

 morphological importance in the evolution of the higher verte- 

 brates. Each canal has an enlarged ampulla at one entrance to 

 the utriculus. The ampullae are at the ventral ends of the two 

 vertical canals, and at the anterior entrance of the horizontal 



