Symbiosis 663 



The hermit crab may live in an empty mollusk shell upon which are 

 placed various types of hydroid coelenterates. The stinging cells (nema- 

 tocysts) of the latter protect the crab, while the sessile coelenterates are 

 advantageously carried from place to place by the crab and secure some 

 foods captured by the crab. 



Fig. 327. — Types of lichens which are plants composed of algae (blue-green or 

 green) and fungi (chiefly Ascomycetes, infrequently Basidiomycetes). The three 

 principal types of lichens are (1) foliose (flat, often leaflike bodies), (2) crustose 

 (hard, often granular crusts on bark or rocks), and (3) jruticose (branched struc- 

 tures which may be erect or hanging). 



The so-called spider crab carries a species of sponge on its back which 

 protects it through concealment and its disagreeable qualities. The ses- 

 sile sponge is carried from place to place by the crab. 



