OBSERVATIONS OF THYONE BRIAREUS. 



277 



ments after being replaced in sea water and gave typical, burrow- 

 ing, feeding and shadow reactions within a few hours. The vis- 

 ceral organs were cast out in only one case and that was after an 

 individual had been left in perfectly fresh water for three hours. 



Table II. 



Results of experiments in which Thyone was placed in mixtures of fresh and sea 

 water. 



As will be seen from the table, animals which were left for 

 twenty-four hours in a solution which consisted of one third sea 

 water and two thirds fresh water, were apparently uninjured ; 

 while individuals which were immersed in fresh water for three 

 hours died. Mr. E. D. Congdon's observations on this species are 

 of interest in this connection. He told the writer that he had found 

 Thyone briarcns at the mouths of rivers in water which was half 

 salt and half fresh, as judged by the specific gravity, but it was 

 never found any farther up rivers than that. 



5. LigJit Stimulation. — Thyone is extremely sensitive to a de- 

 crease in the light intensity and what may be called the " shadow " 

 reaction is one of its most characteristic responses. If an in- 

 dividual is resting quietly in the sand with only the posterior end 

 of the body exposed and the experimenter's hand is passed be- 

 tween it and the window, it at once withdraws the visible portion 

 of the body. This response is of course variable and it may not 

 occur at all or it may be so pronounced that the animal com- 

 pletely disappears beneath the sand. The same withdrawal is 

 induced if a shadow is thrown on the anterior end or even on one 

 tentacle and a particularly sensitive individual was caused to con- 

 tract by extending a pencil over the top of the beaker in which it lay. 



