3l8 FRESH- WATER BIOLOGY 



besides hydra-like forms or hydranths, another type of individuals 

 is produced which become medusae and separate from the colony 

 as free-swimming forms that develop germ cells which in turn pro- 

 duce a generation of individuals of the hydranth type. In other 

 colonial forms the germ cells are formed by individuals that re- 

 main as members of the colony. All the species of Hydrozoa 

 which have a complex colonial organization are with one exception 

 marine. 



In the fresh-water colonial hydroid Cordylophora, many of the 

 individuals or zooids are nutritive and provide food for the colony 

 and by budding increase its size while other individuals form germ 

 cells; there are no medusae formed. Among the obvious structural 

 features in which this form differs from hydra are the following: 

 the tentacles are not hollow but the entoderm forms a core of large 

 cells which occupies all the space enclosed by the ectoderm and 

 mesogloea; the tentacles are more numerous than in hydra and are 

 irregularly distributed; the cuticula is thick and forms a support- 

 ing skeleton for the colony. 



Four genera of fresh-water Hydrozoa form free-swimming me- 

 dusae. Two of these occur in Africa but the two following genera 

 are each known in North America and Europe. 



Edward Potts first discovered the Microhydra and it has been 

 studiM chiefly by him. The hydranth form has no tentacles and 

 it lives independently or forms simple colonies of two or three 

 individuals. The medusae have been seen by him to arise by 

 budding from hydranths but have not been observed when older 

 than a stage attained two or three days after being freed. They 

 have but eight tentacles and no marginal sense organs. 



Craspedaciista was first found in the Regent's Park Gardens, 

 London, England, in 1880, and its only occurrence in North Amer- 

 ica thus far recorded was in Washington, D. C, in 1907 (Hargitt). 

 Only its medusa stage is known with certainty but what is supposed 

 to be the hydranth form is very similar to that of Microhydra. 

 The medusa has more than eight tentacles and has marginal sense 

 organs. 



The hydra is usually found adhering firmly by the base to sub- 

 merged objects over which it moves slowly and may be found at 



