446 HOVEY JORDAN 



When "cornered" by the current it literally stands on its head, a ter- 

 mination of the negative reaction which is extremely unusual among 

 fishes. 4 This high sensitivity suggests at once an explanation of pos- 

 terior and lateral rheotropism, for it may be that stimulation of the 

 lips by the current in the aquarium or cage was so strong as to produce 

 decided irritation, and thus to cause the fish to place that portion of 

 its skin in a less exposed position. 



C. Summary of normal rheotropism 







The foregoing experiments show: 



1. That to a moderate artificial water-current a normal orientation 

 of Epinephelus striatus, in groups or individually, is posterior or lat- 

 eral, as phases of one complete reaction, but almost never anterior. 



2. That the lips are the most sensitive integumentary region, other 

 regions being less sensitive in the following order; tail>, dorsal fin>, 

 side of head > , middle of body. 



3. That the peculiar posterior and lateral reaction to a current is 

 perhaps an attempt to protect from the current the highly sensitive 

 lips. 



D. The end organs concerned in rheotropism 



1. Method of determination. In searching for the end organs con- 

 cerned in rheotropism, it was, of course, necessary to consider all pos- 

 sible sensory cells. My conclusions relative to the significance of equi- 

 libration (semi-circular canals), muscle sense, and pressure sense in 

 these reactions are, for the most part, based upon observations only. 

 The lateral-line organs, eyes and cutaneous receptors, on the other 

 hand, were experimentally eliminated, and each operated fish was care- 

 fully studied to detect any resulting variation of the response from that 

 of the normal individual. It was established by these experiments 

 that the end organs concerned in rheotropism in the hamlet are located 

 in the integument and are probably the tactile corpuscles. 



2. Observations and experiments: a. Observations. When confined in 

 large volumes of still water, groupers are seen usually to lie inactive on 

 the bottom of the tank. In captivity they swim about very little, seem- 



4 It is important to note that the same reaction can be induced by the use of 

 a fine glass rod (tactile stimulation), and also that the variation in regional 

 sensitivity to such stimulation corresponds exactly to that described for stimu- 

 lation by the current. 



