448 HOVEY JORDAN 



tend to substantiate his conclusion that [considerable] pressures do 

 not cause or influence the rheotropic reactions. 



Observations were also made upon the equilibration of hamlets. 

 They were studied in still water and when subjected to a current suffi- 

 ciently strong to cause a 'change in the direction of the dorso-ventral 

 axis. Any differences between the rheotropic response of fishes whose 

 dorso-ventral axis is normal and those in which this axis has been dis- 

 placed would suggest that the organs of equilibrium may be involved. 

 An individual fish when in quiet water usually lies in such a position 

 that its dorso-ventral axis makes an angle' of 10 to 15 with the normal 

 (fig. 4). When this angle was doubled by a current from a glass tube 

 directed against the side of the fish there was no variation in the time 

 required to produce a negative reaction. Moreover the motion which 

 restores the fish to an approximately vertical position usually follows, 

 and never precedes, this rheotropic response; whereas, if the fish is 

 carefully put in the same oblique position by a slow displacement with 

 the hand, instead of by the current, the righting movement takes place 

 much more promptly. It seems, then, that any slight disturbance in 

 the hamlet's equilibrium which the current might cause would neither 

 produce nor in any way affect the observed behavior. This conclu- 

 sion is in accord with Parker's conclusion (6) (p. 203) to which reference 

 has already been made. 



b. Experiments. Lateral-line organs can be excluded from the list 

 of possible rheotropic receptors for two reasons: First, when the cur- 

 rent is directed immediately against the lateral-line canal upon a lim- 

 ited mid-body area, the slow response (thirty seconds) characteristic 

 of regions both dorsal and ventral to the lateral-line, but excluding it, 

 is neither quickened nor retarded. Secondly, a hamlet in which all 

 of the lateral-line nerves had been severed responded normally to the 

 current. This experiment confirms Parker's results (4) (6) from a 

 similar test made upon Fundulus. 



In order to determine whether the visual organs are essential to these 

 reactions, experiments were also performed successively on several 

 individuals after enucleation of both eyes. Fishes thus blinded were 

 subjected to conditions of stimulation identical with those in the tests 

 which were made upon un operated hamlets (fig. 2) and the succes- 

 sive positions which they assumed were recorded. One of these records 

 is reproduced (fig. 5, table 3) for comparison with figure 3, which shows 

 the consecutive orientations of a normal fish in a like environment. 



