430 



Causes and Course of Organic Evolution 



sory epithelium, the other being glandular; this in D. crassus 

 extends backwards beyond the brain as a free tube. In several 

 genera of this order the cerebral organ lies in front of the brain 

 (Tetrastemyna, sp. of Emiemertes, and of Amphiporus); in others 

 it lies at the side, and in still others behind the brain — in which 

 case it attains a great size. In all cases the organ is separate 

 from the dorsal brain mass, from which it receives nerves." 

 (Benhain in Lank. Zool. IV, p. 185). 



A comparison with the embryology and structure of the 

 vertebrate ear leads us to believe that here one has to deal 

 with evolving stages leading toward that organ. The accom- 

 panying diagram, copied from Burger's beautiful work, is 

 suggestive, and can be compared with a figure of the embryonic 

 ear in a mammal. 



c/u. coch. 



s.u.ca. 



du.coch. 



Y.n. 



—du.endo.- 



dn. 



Fig. 16 — a (to left) cerebral organ of Drepanophorus compared as an audi- 

 tory organ with h (to right), the young auditory organ of mammal, o, o' open 

 or closed external orifice; du. coch. ductus cochlearis; s. u. ca. sacculo-utricular 

 canal; utr. utriculus; sac. sacculus; d. n., v. n. dorsal and ventral auditory nerves; 

 du. endo. ductus endolymphaticus. 



It is now well recognized that the ear is innervated by two 

 distinct branches of the auditory nerve, and that it performs 

 a double function. The cochlear branch, distributed to the 

 cochlea and ampullae of the ear, enables the latter to perceive 

 sounds; the vestibular branch that passes to the semicircular 

 canals has for function the maintaining of equilibrium, or, 

 as we would suggest, is geoperceptive or geotactic. 



