320 FRESH-WATER BIOLOGY 



mixture either hot or cold gives sufficiently good results as a fixa- 

 tive, but for certain cytological studies special methods are recom- 

 mended; for these one must consult the literature. 



Occurring most frequently attached to submerged sticks or twigs 

 Cordylophora may also be looked for on the submerged surfaces of 

 walls and piers and also on stems of coarse vegetation. Fixation 

 may be accomplished as with hydra. 



It has recently been shown that the Linnaean systematic names 

 in common use for species of Hydra must be dropped for the earlier 

 ones of Pallas. Recent literature which deals with the results 

 obtained by several investigators who have worked on Hydra 

 shows such conflicting views concerning the status of certain sup- 

 posed species of this genus, that any classification or key dealing 

 with them must be regarded as tentative. The chief difficulty is 

 with Hydra oligactis Pallas (H. fusca L.), which by some is believed 

 to have been applied in the past to two specifically distinct forms 

 while others uphold a contrary view. 



The treatment of the species of Hydra in the following key is 

 based chiefly on the papers of Brauer, Downing, and Koelitz. 



KEY TO NORTH-AMERICAN FRESH-WATER HYDROZOA 



1 (10) Hydranths with tentacles; no free swimming medusae at any stage of 



the life history 2 



2 (9) Tentacles in a circle about the oral end; do not form true colonies; 



have power of slow locomotion. . . Hydra Linnaeus . . 3 



3 (6) Body not definitely stalked; extended tentacles not very much longer 



than the body 4 



4 (5) Green; three kinds of nematocysts; embryonic chitinous membrane 



spherical, with minute elevations; spermaries limited to 



oral third of body; sexual activity more frequent in summer. 



Hydra mridissima Pallas (H. viridis L.) 1766. 



5 (4) Pale yellow, gray, or brown; four kinds of nematocysts, diameter of 



largest 0.0105-0.013 mm.; embryonic chitinous membrane 

 spherical, with coarse branched pointed spines; spermaries 

 only on distal third; sexual activity more frequent in sum- 

 mer Hydra vulgaris Pallas (H. grisea L.) 1766. 



6 (3) Body definitely stalked; extended tentacles much longer than body. . 7 



