MICHIGAN ACADEMY OF SCIENCE. 131 



case was that of one medusa swiniining through the hglit area 14 times 

 in 72 seconds Avithout reaching toj) or Ijottom. When the aquarium was so 

 dark that I coidd not see the medusae, sudden ilhimination showed that 

 they had attached l)y their tentacles to the side of atiuarium. This reaction 

 has been often ol)served in connection with other violent stimuli and will 

 be reported at another time. Thus the presence of light seems necessary 

 for the regular up-swimming activity of Gonionemus and gravity may 

 direct it. 



In regard to the part light plays in the surface reaction Yerkes [(3) page 459] 

 says, "although light seems to be one of the important conditions for this 

 reaction, it may occur in the absence of light." From the experiment just 

 cited I would conclude that the chief importance of light is in bringing 

 Gonionenms to the surface. Morse [(3) p. 451] says "The cause for reaction 

 is not evident." Later [(4) p. 686] he explains the "inversion" by assuming 

 that the bell is thrust above the surface unevenly. Yerkes [(5) p. 255] 

 has given a similar explanation. Gravity, no doubt, causes the inversion, 

 not as a stimulus but because the apex of the l^ell is heavier. 



Now as the inversion at the surface is preceded by inhibition of contraction 

 the question arises as to the cause of this inhibition. Yerkes (5, p. 281) and 

 Morse (3, pp. 450, 451) have observed that medusae do not turn (invert) 

 M'hen made to swim up against a heavier substance than air, such as board, 

 a glass plate, or a layer of olive oil, but they continue to swim against these 

 layers until exhausted. From these results I believe that contact of the 

 apex of the l^ell with air normally causes cessation of movement (inhibition). 

 This could be proved by holding a layer of imprisoned air some distance 

 below the surface of the water (7). Light is probably not the cause [Morse 

 (3) p. 451] and therefore no further discussion is warranted here. 



SUMMARY. 



1. Change of light intensity, not light per se, is a normal stimulus for 

 reactions of Gonionemus. 



2. The medusae do not usually direct their movements to favorable 

 locations but continue to make random movements until they come into an 

 optinmm environment. 



3. Relative intensity of light and not ray direction determines the place 

 of rest. 



4. Intense light may direct the medusae causing them to turn away from 

 its source. Ordinary light is important for the up-swimming activity, 

 though not directive, this being due to gravity. 



5. Contact of the bell with air may cause the inhibition of movement 

 and relaxation allowing inversion of the bell by gravity. 



Detroit, Mich., A])ril 1909. 



3. Jour, of- Cornp. Neurol. P.svchol.. lOOG, Vol. XVI. 



4. American Nat., 1907, Vol. XLI. 



~^. American Jour. Physiol., 1903. Vol. IX. 



7. This has since been verified, but another explanation will be given in another place. 

 (Biol. Bui.. 1909, Vol. XVII, p. 354.) 



