lt)b 'I HE AMERICAN NATURALIST. [Vol. XXXVII. 



in the skin or in tubes that lie immediately under the skin and 

 whose courses are indicated by numerous openings, the lateral- 

 line pores. In many fishes three such lines of pores can be 

 recognized on the side of the head, one above the eye, a second 

 immediately below the eye, and a third on the lower jaw. From 

 the union of these three near the back of the head, a fourth line 

 passes posteriorly along the side of the fish, the lateral line, from 

 which the whole system takes its name. 



Modern embryology has shown that the ear is closely related 

 to the lateral-line system. The deep situation of the ear-sac at 

 first sight seems to preclude this, but the ear-sac does not form 

 in this situation. In fishes, as in higher vertebrates, this sac 

 forms as a pocket of skin pushed into the head as it were from 

 the outer surface and in fishes it can be shown that the skin 

 which is thus infolded is a portion of the lateral-line system. 

 After the sac has been formed, it sinks into the deeper part of 

 the head, generally loses its connection with the outer skin, and 

 gradually takes on its final complicated shape by producing semi- 

 circular canals, etc. Thus the internal ear may be regarded as 

 a modified part of the lateral-line system. This system in turn 

 develops from the skin, and when its organs lie ii 



lies, the tube 



he in tubes, as they 

 i groove-like depres- 

 inus the lateral-line organs are specialized 

 sense organs from the skin. 



These facts suggest at once interesting relations between the 

 three sets of organs mentioned ; for, as the lateral-line organs 

 may be regarded as derived from the undifferentiated sense 

 organs of the skin, so the ear may be conceived to have been 

 derived from the lateral-line organs. Thus, we are dealing with 

 what may be called three generations of sense organs : the skin 

 representing toe first^ generation, and giving rise to the lateral- 

 well with what I have been able to make out about their func- 

 tions in the killifish. It has already been shown that only such 

 fishes as have their ears intact respond to the sound of a tuning 

 fork. Consequently we may conclude that such sounds do not 

 stimulate either the lateral-line organs or the skin, but that they 

 are a stimulus appropriate to the ear. 



