240 AYERS. [Vol. VI. 



relations, even in the adult forms of the present day, the other 

 has been modified so as to form the internal ear in the manner 

 already described. 



The lateral line organs take up their wave motions from a 

 more limpid fluid than the ear organs, since the endolymph 

 contains a quantity of mucin and earthy matter in solution, 

 which not only increases its specific gravity, but gives it a 

 certain viscidity. This denser medium of the internal ear is 

 more favorable to the rapid transmission of auditory stimuli 

 than water, and it may be that in this fact we have an explana- 

 tion of the origin of the mucous canals of Elasmobranchs. 



The lateral line organs also perform an important function 

 indirectly as organs of equilibration, since they convey to the 

 centres of co-ordination information of the direction in which 

 matter in space surrounding the animal is moving. 



Suppose a fish to have its body rotated about the long axis ; 

 it is evident that this motion will cause two currents to be pro- 

 duced, setting in opposite directions. Thus, if the rotation 

 is from the right to the left, or with the hands of the watch, 

 the current on the left side will be upward and the current on 

 the right side will be downward, when judged from the fish's 

 standpoint ; for though the water moves not at all, the fish 

 would refer the movement to the water, since the sensation of 

 motion would be taken up from or caused by the water. 



These organs are also auditory in a sense ; for they are like 

 the special auditory organs in their structure and central nerve 

 relations, and since the ear organs have been derived from, and 

 what is more important still, since at least three of the sense 

 organs of the auditory apparatus retain all the characteristics of 

 the superficial organs, we may reverse the line of argument, 

 and say that all the external organs are primarily auditory in 

 function though in an undifferentiated degree. 



After a brief resume of the main historical features of the 

 semicircular canal question, we will pass to a consideration of 

 the function of these famous tubes and their inclosed sense 

 organs, the cristse acusticae, in the light of new facts brought 

 forward by a series of experiments recently carried out by 

 Professor Steiner of Heidelberg University, and of the new 

 morphological considerations already given in this paper. 



The semicircular canals have been the subject of untold num- 



