THE HYDRA AND TTN 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 



-i i ,1 11 FIG. 1!H. Stylactis, a tubularian 



rocks beneath seaweed, and hydroid> gl . ^ ugou asn:iiL Nat . 



looking like delicate white size. Photo, of living animal by 



threads (Fig. 196). Observed 



FIG. 195. Hydractinia, a tubularian hydroid. a, 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. 



os, a spit. 



