The Senses and the Nervous System 109 



just possible that the automatic equilibrating system is not 

 indispensable to us, who live on land and have a strong 

 force of gravity to tell us what is up and down and things 

 to look at to tell us which is right and left. Perhaps we 

 should never have developed such a system by ourselves. 

 Perhaps it is only because the animals from which we are 

 descended lived in the unmarked waters that we have it. 

 And yet, if we did not have it, we might have more difficul- 

 ties flying our airplanes than we do. Perhaps it is only be- 

 cause we come from fish that we are able to learn to fly. 



LATERAL LINE 



In close connection with the ear is that mysterious organ, 

 the lateral-line canal. It is an organ found only in animals 



.Skin 

 Scale 

 -Ciindl 



"YC'*"' ' "^V T" " ' ' ""^VT^ " Sense Organs 



. , ' Netve Trunk 



Figure 14. DIAGRAMMATIC REPRESENTATION OF THE 

 LATERAL-LINE SYSTEM 



dwelling in the water. It is visible on most fish — with the 

 exception of the gobies, and with the more notable excep- 

 tion of the guppies, sword-tails, platies, and other killifishes 

 — as a line extending along the side from the gill opening to 

 the tail. It consists of a tube beneath the skin, filled with 

 mucus, and opening to the exterior at intervals through pores 

 which may pass either through the scales or between them. 

 In most fish it continues on to the head, where it breaks up 

 into several branches, but these are generally buried in the 

 surface bones of the skull and therefore invisible. Beneath 

 the canal runs a nerve which at frequent intervals sends out 

 branches to it, each branch having a sensory ending. In some 



