238 MARY O. McGINNIS 



By covering the cylinder with a black cloth through which 

 some light penetrated, a dim illumination was produced. Under 

 these conditions the animals remained at the bottom. 



The above record indicates that in darkness Branchipus shows 

 marked negative geotropism. We may conclude that Branchipus, 

 like Cyclops (Esterly '07), is positively geotropic in the light, 

 while in darkness this response is changed to a negative one. 



Parker ('02) and Juday ('04), in their investigations concerning 

 daily depth migrations with animals which are negative to ordi- 

 nary daylight but positive to dim light, account for their rise to 

 the surface at night, by the combined effect of negative geotropism 

 and positive phototropism to weak light. The descent in the 

 daytime, they attribute to negative phototropism in strong light. 

 This response to strong light overcomes the negative response 

 to gravity. 



It is probable from the present experiments with Branchipus, 

 and those of Esterly with Cyclops, that gravity, as well as light, 

 does have an important influence upon the depth migrations of 

 these crustaceans; but, at least as far as Branchipus is concerned, 

 not in the manner suggested by Parker and Juday. It seems 

 more probable, in the case of Branchipus, which is always posi- 

 tively phototropic, that light has an effect in addition to its 

 directive and kinetic effects, so that through some internal change, 

 the animal is made positively geotropic, and hence goes down in 

 the daytime. Or it may be that darkness furnishes a stimulus 

 which renders the animal negatively geotropic, and so causes it 

 to rise to the surface in the absence of light. It appears therefore 

 that light (or darkness) brings about a reversal of response to 

 gravity, just as mechanical influence brings about a reversal of 

 response to light, as shown by Towle ('00) and as Loeb ('04) has 

 shown chemical influence to cause a reversal of response to light. 



When this investigation was undertaken it was intended to 

 check and supplement the results obtained in the laboratory by 

 field observations on the daily life of Branchipus in nature. On 

 account of the unusual drouth early in the spring of 1910 in the 

 vicinity of Urbana the Branchipus season was shortened b}^ about 



