784 THE AMERICAN NATURALIST. [Voı. XXXIX. 
9. Pandorina sometimes forms a green scum on the surface 
of the water amongst water lilies, or on the lily leaves. 
IO. Hydra occurs most abundantly on the following plants: 
duckweed, Riccia, Ceratophyllum, Elodea, watercresses. Two 
methods of collecting may be used. (a). Method best adapted 
to duckweed and Riccia: bring in the material with enough 
water to keep it moist. Fill a large number of bacteria dishes 
nearly full of clear water. Place in each dish a small quantity 
of the plant material — scarcely enough to cover the surface of 
the water on which it floats. Place each dish so that one side 
will be exposed to the light. A day or two later hydras will be 
found in the extended condition, clinging to the side of the dish, 
especially the lighted side, to rhizoids of plants, or to dead plants 
and sediment at the bottom. They may be removed from the 
sides of the dish with a pipette, or if clinging to plants, the lat- 
ter may be removed with forceps. Place the hydras in a covered 
dish of clean water with a small amount of actively growing plant 
material. Here they will keep indefinitely and may readily be 
found when wanted. It is not safe to leave them in the original 
dishes, as there may be present crustaceans which will eat them, 
or an excess of vegetable matter may cause the water to become 
foul, which is unfavorable for Hydra. However, the dishes 
should be inspected daily, for sometimes the material will yield 
Hydra indefinitely. 
(b). When Hydra is to be obtained from large plants like 
Ceratophyllum and Elodea, the best method is as follows: place 
a considerable quantity of the plant material in each of several 
"large-bacteria dishes and cover it with water. Ina short time 
the water becomes foul through the decomposition of vegetable 
matter; the hydras then loosen their hold upon the vegetation 
and may be found floating at the surface of the water. They 
must be picked off at once with à pipette and removed to clean 
water. 
Hydra will reproduce readily by budding if kept in a fairly 
"m room; but the optimum temperature is not very high. 
It is said that if Hydra is kept in the dark at a slightly lower 
temperature than usual for several days, this will favor the 
formation of spermaries and ovaries. Rarely, a mature fertil- 
ized egg in the winter Condition may be found. 
