208 ZOOLOGY 



lophora produces its young in special capsules, called 

 gonophores, which are rudimentary jelly-fishes (Fig. 193). 

 The young become free when they have gained an elon- 

 gated, cylindrical form. There are other fresh-water 

 species which have free jelly-fishes. In this country we 

 have a species, Microhydra l Ryderi, hitherto known only 

 about Philadelphia, whose hydroid stage is extremely 

 small and bears no tentacles. The jelly-fishes are set 

 free during July. Fresh-water jelly-fishes have also been 

 described from Lake Tanganyka, Africa, and from a tank 

 at Regent's Park, London, to which they had doubtless 

 been imported on plants. It is very probable that all 

 fresh-water hydroids which produce jelly-fish have, geo- 

 logically speaking, recently come from the sea. Hydra, 

 however, is probably a long-established fresh-water species. 



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

 ones, very numerous. The Hydrocorallidae 2 are peculiar 

 in that they secrete a great amount of calcareous sub- 

 stance, so that they were formerly regarded as belonging 

 to the typical corals (Scyphozoa). They may be easily 

 distinguished by the absence of radial septse in the cups 

 occupied by the hydroid. Here belong the millepore 

 corals of Florida. 



The Tubularidae 3 include some hydroids of large size, 

 single individuals of Tubularia becoming six inches long. 

 Other species grow on gastropod shells which are occu- 

 pied by hermit-crabs (Figs. 194 and 195). So thickly do 

 they grow that they make a plush-like covering on the 

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



1 Small Hydra. 



2 Combining the qualities of Hydra and the corals. 



3 From tubulus, a little tube. 



