Evolution of Fishes from Invertebrates 83 



paired organs developed along the postero-lateral sides of 

 the head. Each in its simplest state is 'a mere groove in 

 the epidermis not extending deeper than the basement mem- 

 brane; it is lined by ciliated cells, and at the bottom are 

 large gland cells; while the organ is supplied by nerves 

 from the dorsal ganglion of the brain. In Carinella rubi- 

 cunda and others the groove becomes an oblique canal, the 

 blind end of which is surrounded by a mass of ganglion 

 cells, lying outside the cutis. In the higher forms the 

 canal penetrates deeper into the body as far as the brain. 

 The gland cells and the associated nerve tissue increase in 

 amount, and the canal becomes differentiated into two 

 regions — an extra-ganglionic 'lateral canal' and an intra- 

 ganglionic 'cerebral canal' (c) which frequently termi- 

 nates in an enlarged sac. In Drepanophorus the cerebral 

 canal is quite exceptional, in that it bifurcates, one branch 

 terminating in a sac with sensory epithelium, the other 

 being glandular; this in D. crassus extends backwards be- 

 yond the brain as a free tube. In several genera of this 

 order the cerebral organ lies in front of the brain {Tetra- 

 stemma, sp. of Eiineviertes, and of J m phi poms) ; 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 re- 

 ceives nerves.' (Benham 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 

 accompanying diagram, copied from Burger's beautiful 

 work, is suggestive, and can be compared with a figure 

 of the embryonic ear in a mammal. 



"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 branchy dis- 

 tributed 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 main- 

 taining of equilibrium, or, as we would suggest is geoper- 

 ceptive or geotactic. 



