HYDRA 267 



jellyfish have, geologically speaking, recently come from the 

 sea. Hydra is, however, probaljly a genus that has been es- 

 tabhshed in fresh water for a long time. 



The marine hydroida are, in contrast to the fresh-water 

 ones, very numerous. The organ-pipe corals (Hydrocoralhdse^) 

 are peculiar in that they secrete a great amount of calcareous 

 substance, so that they were formerly regarded as belonging 

 to the typical corals (Scyphozoa, see page 275). They may 

 he easily distinguished by the absence of radial septje in the 

 cups occupied by the hydroid. Here belong the millepore 

 corals of Florida. 



The tubulurian hydroids (Tubularidse ^) include some Hy- 

 droids of large size, since single individuals of Tubularia may 

 become six inches long. Other species grow on gasteropod 

 shells which are occupied by hermit-crabs (Figs. 255 and 256) . 

 So thickly do they grow that thej make a plush-like covering 

 on the upper part of the shell, and they have the curious habit 

 of building out the lip of the shell so as to enlarge the aperture. 

 Because they do this the hermit-cral^, as it grows larger, is 

 not forced to exchange the shell for a larger one, and so does 

 not leave the hydroids on the cast-off shell to roll about on the 

 beach and perish. 



Of the bell-hydroids, Campanularidse,' one of the com- 

 mon representatives is Obelia,^ which may be found at low 

 tide hanging from rocks beneath seaweed, and looking like 

 delicate white threads (Fig. 257) . Observed under the micro- 

 scope each stem appears as a series of heads called hydranths 

 placed in zigzag fashion, one beyond the other. Another 



^ Combining the qualities of Hydra and the corals. 

 ^ From tuhulus, a little tube. 

 3 campanula, a little bell. 

 * obelos, a spit. 



