Mutualism with Continuous Contact 



369 



Certain cockroaches and termites can digest wood only with the aid 

 of a special type of flagellate that is harbored within their guts. 

 Symbiosis in these instances is mutually beneficial and obligatory for 

 both parties. Some of the bacteria living in the intestines of animals 

 also produce various B vitamins and other special materials. 



Another manifestation of mutualism in which the symbionts are in 

 permanent contact, but one in which the contact is entirely external 

 is the attachment of certain marine sponges and coelenterates to the 

 shells of crabs. The attached animal benefits by being carried about 

 to fresh feeding areas and by avoiding being stranded in the tidal 

 zone or in stagnant water, as well as by obtaining fragments of food 

 from the meal of its host. The crab, for its part, is camouflaged to 

 some extent by the presence of the attached animals on its back and 

 is often protected by them from attacks by its enemies. 



Fig. 10.4. Three sea anemones attached to the shell of a hermit crab, illustrating 



mutually beneficial symbiosis. Note the growth of the foot of the anemone over 



the surface of the shell. ( Modified from Borradaile, 1923. ) 



The classic example of this type of partnership under the sea is 

 furnished by the sea anemone Adamsia palliata which grows on the 

 shell of the hermit crab Enpagurus prideauxi (Fig. 10.4). Further 

 interspecific relations result in this instance from the fact that the shell 

 inhabited by the hermit crab is the abandoned house of a snail and 

 from the fact that the same shell may also furnish the abode of the 

 annelid worm Nereis. The hermit crab starts the enterprise by ob- 



