THE BEHAVIOR OF BROOK TROUT EMBRYOS 49 



3. Reaction to Mechanical Jars 



Possibly the first stimulus to which a developing trout reacts 

 is that of mechanical jars. Often before the embryo has com- 

 pletely left the shell, the shaking of the dish or currents in the 

 water cause it to contract, and if the tail is free, to swim about 

 with the head still encased. This sensitiveness to mechanical 

 vibrations continues throughout the larval life. Even so slight 

 a vibration as that caused by blowing on the water makes the 



fish dart about. 



4. Reactions to Touch 



The sense of touch is well developed when the trout hatches* 

 but it is impossible to predict what response will be given to 

 a stimulus in any particular part of the body. Embryos in 

 nearly all stages of development from the time of hatching until 

 the yolk sac was lost, were touched systematically with a slender, 

 pointed stick, and with bristles to determine whether one part 

 of the body was more sensitive than another. I found, as 

 Paton ('07) did, that although it is very difficult to localize the 

 tactile areas, the head is much less sensitive than the body, and 

 the eye is quite insensitive. Judged by such responses, the tail 

 seems to be the most sensitive part of the body. It was found 

 that the trout avoided a brush much more vigorously than they 

 did a pointed stick or a single bristle. This is probably because 

 the brush stimulates them at more points. 



If touched persistently the embryos swim about, turning 



rapidly in all directions. When they are eventually fatigued, 



the responses become less marked, the trout may then merely 



move its tail or increase the rate of movement of its fins. It 



may even become absolutely quiet for a time, after which the 



reactions take place as before. The young trout show some 



measure of adaptability, for they grow somewhat accustomed 



to being handled, and particularly in the case of the oldest fish 



(three months old), those which had been repeatedly picked up 



were slightly easier to catch than those which had never been 



touched. 



5. Reactions to Current 



Since Brook Trout live in swift flowing streams, one would 

 expect them to react to currents. In order to test this matter. 

 a trough was constructed in which they could be tested. The 

 apparatus was fifty inches long by two and one-fourth inches 



