THE HYDRA AND ITS ALLIES 



209 



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

 aperture. They do this in order that the hermit-crab, as 

 it grows larger, shall not be 

 forced to exchange the shell 

 for a larger one, leaving the 

 hydroids on the cast-off shell 

 to roll about on the beach 

 and perish. 



Of the Campanularidae, 1 or 

 bell-hydroids, one of the 

 common representatives is 

 Obelia, 2 which may be found 

 at low tide hanging from 

 rocks beneath seaweed, and 

 looking like delicate white 

 threads (Fig. 196). Observed 



FIG. 194. Stylactis, a tubularian 

 hydroid, growing on a*snail. Nat. 

 size. Photo, of living animal by 

 W. H. C. P. 



FIG. 195. Hydractinia, a tubularian hydroid. , colony growing on gastro- 

 pod shell inhabited by a hermit-crab ; b, bit of colony enlarged ; dz, modified, 

 tactile, individuals ; s, reproductive individuals ; sp, spines. From Parker 

 and Haswell. 



campanula, a little bell. 



6/3eX6s, a spit. 



