CCELENTERATA 



187 



animals and plants captured by the tentacles. The attached 

 forms flourish best in the comparatively shallow water near the 

 shore. Food is especially abundant in such regions and hence 

 the passive animals are more successful here than elsewhere. 

 Hydractina (Fig. 85) and even the sea-anemone form interesting 

 partnerships with the hermit-crab. The polyps may cover por- 

 tions of the shell occupied by the crab, while the polyps doubt- 

 less profit by a share of the food broken to pieces by the crab, 



Fig. 85. 



-r. 



Fig. 85. Hydracnna echinala, after Hincks. c, the eanosarc, forming an incrustation over the 

 object on which it lives; n, nutritive polyps; r, reproductive polyps, bearing buds in which are ova; 

 (. tentacles. 



Questions on the figure. — How many types of individuals seem to be repre- 

 sented? What evidence of budding do you see in the species? What is the cceno- 

 sarc? What is its nature in Hydractina ? What can you find concerning the habits 

 of the members of the genus ? How does this colony compare with that in Fig. 86 ? 



as well as by the change of place as the crab moves about in 

 search of food. Some anemones have living algas in their ento- 

 derm cells which seem to help supply the animal with oxygen 

 in return for foods of other kinds. 



Nearly all the coelenterates are marine. A few species oi 

 Hydra, one parasitic form, Polypodium, and a few fresh-water 



