Jackson, the Amphipod Hyalella Knickerhockeri. 
55 
nearly as I can observe, within an hour or an hour and a half after 
copulation. I have not determined definitely the length of time 
they are in the sac before hatching. I succeeded in carrying only 
one pair through complete breeding experiments and here my 
observations were in many ways incomplete. This pair was iso¬ 
lated April 2ist, at 3:00 P. M. in a glass dish in which were 
stones, mud and algae, the water in the dish being a trifle over 
an inch deep. The male was carrying the female at the time of 
isolation, and continued to carry her until the next day, April 22nd, 
when he released her at 11140 A. M. Between 12:10 P. M. and 
2 :oo P. M. the female molted. I was not present during her molt¬ 
ing, but when I returned at 2 :oo P. M. she had completed it, and 
was inactive, clinging to the algse. Twelve minutes later the male 
found her, by his usual hit or miss method, and carried her until 
2 :2i P. M. He then again released her but took her up again at 
2 :27 P. M.; he carried her until 2155 P. M., and as nearly as I 
could judge, copulation took place between 2 140 and 2 155 P. M. 
About an hour later the eggs were deposited into the sac, and at 
4:10 P. M. the male again carried the female. He continued to 
carry her intermittently until April 30th; sometime between this 
date and the morning of May 3rd he died. Most of the time, 
while the eggs were in the sac, the female was very inactive, hiding 
among the algse or under stones. Eggs were still in the sac May 
nth at 10:00 A. M., but on the morning of May 17th the young 
had left the pouch and were swimming in the water. From these 
observations we see that the eggs were in the pouch twenty-five 
days, at least, before they hatched. After the young had left the 
pouch, the mother seemed less positively thigmotactic than usual, 
and in reality was more negatively sensitive to contact stimuli 
than were males. This period of excitability lasted for four or 
five days, when she gradually assumed her normal condition. 
food and feeding. 
Hyalellce in their natural habitat feed almost entirely upon 
protozoa, and unicellular and filamentous algae; they also eat 
Ceratophyllum to a small extent. Although they are often abund¬ 
ant where Potamogeton is growing, I have never discovered them 
feeding upon it, nor have I been able to induce isolated specimens 
to eat it. Their method of feeding is peculiar; they pasture as it 
