118 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Since females can be rendered negatively phototropic for a period of almost 
two hours by T handling them with a pipette, it might be inferred that this is due to 
simple mechanical stimulation. That such is not exactly the truth is seen in the 
fact that female Labidocerse agitated in the water do not become negative even when 
this process is several times vigorously repeated, although these same animals easily 
change on being handled with a pipette. Apparently, then, it is a very special form 
of mechanical stimulus that is necessary to call forth this reversal. 
Pipette experiments with five males, which in the beginning were almost indif- 
ferent or only slightly negative to light, resulted in making all distinctly negative, a 
condition which lasted at most an hour and a quarter. 
Although certain forms of tactual stimulation are a means of making both males 
and females move away from a source of light, even, as 1 found by experiments, down- 
ward through the water, they can not for a moment be regarded as the efficient cause 
in the downward movements of the Labidocerse each morning, for the animals are quite 
as much open to agitation at other times of day as in the early morning. 
Notwithstanding that this change has probably no connection with the daily 
migrations, it may have, a very important connection with the pairing habits of the 
animals. As is well known, the right antenna of the male is modified to form a 
grasping organ, and by means of this the male seizes the female in pairing and holds 
her till he has deposited a spermatophore on the posterior part of her body. I have 
observed pairing among Labidocerse a number of times, and it has alwaj's taken 
place between individuals at the top of the water. After the male has seized the 
female, the pair plunge about in a most violent manner, and it has occurred to me 
that the mechanical stimulation thus produced might be sufficient to render both 
negatively phototropic, so that after release they would move downward to the water 
where fewer individuals were. This operation, if it really takes place, would have 
the effect at least of removing from the pairing individuals any two that had recently 
been in union. After this idea occurred to me, I observed only one pair in copula- 
lation. These were removed from the general aquarium to determine whether after 
separation their light reactions had changed. After separation, however, both male 
and female exhibited their usual phototropic responses, no change having taken place, 
but on examining the female no spermatophore was found on her, and possibly the 
operation was incomplete in respect to light as it was in sexual matters. 
Attempts were next made to ascertain whether the continuance of light action 
or a difference in its intensity could induce changes in the Labidocerse. Groom and 
Lovb (1891) observed that the free-swimming larvae of a barnacle were positively 
phototropic in the morning, but that before noon and without other changes than 
continued light action they became negative. These in the course of the night all 
became again positive. Thus the continued action of bright light gradually made 
the larvse negative, of dim light positive. 
To ascertain the effect of this form of continuous light action on Labidocerse, I 
put five males in one large jar of sea water and an equal number of females in another. 
These were exposed to diffuse daylight in the laboratory, as in Groom and Loeb’s 
experiment, the jars having been set up August 12. On August 20 one male died, 
the other four succumbed between August 2d and 26. One female died August 2d, 
