506 THE AMERICAN NATURALIST. [Vou. XXXIII. 
In each of the one hundred and twenty-four times a hydra 
was placed in the field of the microscope it immediately under- 
went violent contraction, followed by speedy recovery of the 
elongated form. In four cases a response of this kind was 
noted in five seconds from the time the hydra was placed in the 
light. In two cases a period of forty seconds elapsed before 
any response was noted. 
This response was accompanied by the discharge of a number 
of the nettle cells on the tentacles. In the great majority of 
cases the loss of the ectodermal cells began about the time when 
the animal regained its elongated form. Usually all of the 
ectoderm was lost, but in a few individuals a small number of 
cells would adhere to the upper part of the animal. In one 
case the ectoderm was completely lost in one minute, while 
another required eleven minutes for its removal (see table, also 
Fig. 1). 
TABLE OF RESPONSES OBTAINED. 
Total number of individuals, 124. 
First response : 
Time in seconds, 5 ro 15 20 25 370 35 go 
No. of individ, 4 71 2 110 a8 2 
Lost ectoderm : 
Time in minutes,7 27 4¢ 567891041 
No. of individ, 106 10 80712313 1 
Ectoderm regenerated : 
Time in days, 9 fo I7 12 13 14 15 16 17 18 £9 20 21 22 
No. of individ, © 4 7 7 10 20 go 20:12 3° 1° ù o 
23 24 25 20 27 28 29 30 31 32 33 
t S EO Oe Oe Poe O f f 
Time lived after second trial : 
Time m days, F4 5 6 7 8 g 
No. of individ., 2 2 10 42 44 14 10 
The first response is evidently due to the concentration of 
light upon theanimal. An interesting point about the response 
to the light stimulus is the grouping of the individuals about 
two modes (Fig. 2). One set responds quickly, and tends to 
gather about a mode of ten (seconds). The other set requires 
