508 THE AMERICAN NATURALIST. {VoL XXXIII. 
No experiments were made to determine what light rays were 
responsible for this, excepting that it was noted in making a 
photo-micrograph, using light of long wave-length, that the 
phenomenon did not occur; but if rays of short wave-length 
were used, it was impossible to obtain a sharp photograph, 
owing to the sloughing of the hypodermis. Microscopical 
examination showed that the sloughing had occurred, and it 
was also seen on the focusing screen of the camera. Beyond 
this I did not determine the action of the rays of different wave- 
lengths, although I believe this phenomenon is in some way 
related to rays of short wave-length. 
After the removal of the ectoderm by this means, the hydra 
was very unresponsive to stimuli. It remained in an elongated 
condition even when handled violently. Before twenty-four 
hours had elapsed it had contracted into an oval mass, with the 
tentacles almost entirely retracted; but it recovered its normal 
expanded condition in three or four days. 
It is interesting in connection with this to note the regener- 
ation of the ectoderm. Each hydra, after having been subjected 
to this treatment, was placed in a small bottle and given plenty 
of fresh water until it recovered. The most rapid recovery took 
place in nine days, while the slowest required thirty-three. The 
great majority recovered in from fifteen to seventeen days (see 
table). I have made no histological study of the regeneration 
of the ectoderm, but observed it with considerable constancy in 
the living animals, and found that it gee first about the 
oral end. 
If a hydra after complete recovery, that is, after complete 
regeneration of the ectoderm, was again subjected to the same 
treatment, the second response to light would occur in about 
the same time as before, and usually fall in the same modal 
class. This was true also for the loss of the ectoderm. It was 
noted, however, that some of the individuals that responded 
quickly to light and to the stimulus that removed the ectoderm 
in the first trial showed a near approach, or completely con- 
formed to the second modal class in the second trial. 
I was not able to obtain a second regeneration. The hydra 
did not recover at all, but remained as if stupefied, moving only 
