COELENTERATA 41 



This pinkish covering' is found on shells and 

 occasionally on pebbles throughout the entire coastal 

 waters of New Jersey, as well as along the New 

 England coast, The shells covered by Hydractinia 

 are frequently inhabited by Hermit Crabs (Parugus). 



Under the microscope or fairly powerful lens 

 the rather complicated organization of this colony 

 can be observed. There is a root-like net work 

 covering the surface of the shell; from this covering 

 various polyps or individuals of the colony arise. 



Hydractinia echinata Fleming- 

 Fig, l 



Hydractinia echinata Fleming 



There are three types of polyp: one for feeding, one 

 for reproduction and a third type for protection, 

 armed with stinging cells or nematocysts. 



The peculiar association between the Hydractinia 

 and the Hermit Crab in the shell is known as com- 

 mensalism, which is a sort of partnership formed 

 for mutual benefit, The crab benefits by being partly 

 concealed by the hydroid and by the stinging cells 

 of Hydractinia which protect it from enemies and 

 paralyze its prey; in exchange the hydroid obtains 

 transportation from the crab, thereby being able to 



