HYDRA AND OTHER FRESH-WATER HYDROZOA 



3 2I 



7 (8) Gray, brown, or reddish; three kinds of nematocysts; diameter of 

 largest less than 0.0105 mm. ; embryonic chitinous membrane 

 spherical, with very short spines; spermaries on any part 

 of body except the stalk; sexual activity more frequent in 

 winter Hydra oligactis Pallas (H.fusca L.) 1766. 



By some it is claimed that H. oligactis is strictly dioe- 

 cious and is in this way distinct from the following 

 species. 



FIG. 584. Hydra oligactis. (a) Nematocysts. (b) Embryonic 

 chitinous membrane. X 47- (After Brauer.) 



8 (7) Gray or brown; four kinds of nematocysts, diameter of largest less 

 than o.o i mm.; embryonic chitinous membrane plano-con- 

 vex, with only convex side covered with spines; spermaries 

 limited to the oral third of the body; sexual activity more 

 frequent in autumn Hydra polypus Linnaeus 1758. 



Besides the differences between H. oligactis and H. polypus 

 mentioned above the latter is said to be somewhat smaller and 

 to have somewhat shorter tentacles than the former. By some 

 the validity of any of the differing characters mentioned above 

 is disputed, with the possible exception of the difference in the 

 number of different kinds of nematocysts. 



H. pallida Beardsley, a very pale form in Colorado, and H. 

 corala Elrod, a very large red form in Montana, may prove to 

 belong to the species listed above, as similar variations of them 

 are known to occur in Europe. 



FIG. 585. Hydra polypus, (a) 

 Nematocysts. (b) Embry- 

 onic chitinous membrane. 

 X 36. (After Brauer.) 



9 (2) Tentacles irregularly scattered on the body of the hydranth; form true 

 colonies Cordylophora Allman. 



But one species, C. lacustris Allman, which 

 occurs in fresh water near Philadelphia, Pa., 

 and near Woods Hole, Mass. It has recently 

 been found in the Illinois River at Havana, 

 and by Mr. W. Donaldson in the Mississippi 

 River at Granite City and East St. Louis, 111., 

 in the Arkansas River at Little Rock, Ark., 

 and in the Red River at Shreveport, La. 



FIG. 586. Cordylophora lacustris. (a) A branch from 

 a colony. About twice as large as is common in fresh 

 water, (b) Female reproductive zooids with embryos 

 in different stages of development. X 20. (After 

 Schulze.) 



10 (i) Hydranths without tentacles; free swimming medusae are formed. . n 



