162 



BIOLOGY: GENERAL AND MEDICAL 



resembling the vertebrate eye, but having certain essen- 

 tial differences, for the retinal nerve cells are directed 

 toward the centre of the globe and are outside of the 

 pigment layer, while in the more perfect vertebrate 

 organ the nerve endings are directed away from the 

 centre and the pigment layer of the retina is outside. 



As the structure of the eye increases in perfection the 

 number of nervous elements increases greatly, and the 

 complexity of the central nervous system is increased 

 both by the increased number of fibres it receives and 



External aperture 



FIQ. 63. Section through the otocyst of arenicola. (After Ashworth and 



Gamble.) 



the number of cells with which they communicate, so 

 that the new centres, optic lobes, and optic thalami make 

 their appearance. 



Hearing. In this sense we doubtless have to do with 

 a specialization of thigmotropic irritability to vibrations 

 set up in the media in which the organism lives. The 

 inception of the organs by which such vibrations are 

 originally recognized is unknown. The first organs 

 that can be definitely made out are found among the 

 ccelenterates. In certain jelly-fishes minute vesicles 

 are found situated at the edge of the disc, each contain- 



