i86 



ZOOLOGY 



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 cover up the 

 shell occupied by the crab, thus concealing it from its enemies 

 and its prey. In return the polyps doubtless profit by a share 



FIG. 85. 



FIG. 85. Hydractina echinata, after Hincks. c. the cctnosarc, forming an incrustation over the 

 object on which it lives; , 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? 



of the food broken to pieces by the crab, as well as by the change 

 of place as the crab moves about in search of food. Some anem- 

 ones have living algae in their entoderm cells which seem to 

 help supply the animal with oxygen in return for foods of other 

 kinds. 



Nearly all the ccelenterates are marine. A few species of 

 Hydra, one or two partly parasitic types, and a few fresh- water 



