Handbook of Paleontology 113 



places provided by colonial forms of corals and sponges 

 to many animals. When two forms live together with 

 mutual benefit to both, the association is known as com- 

 mensalism. The Hermit Crab- Sea Anemone association 

 is an example of this. The Hermit Crab carries the 

 anemone from place to place on its shell so that it is able 

 to secure a more abundant and varied food supply besides 

 getting the scraps of food the crab lets drop, and in 

 return, the Hermit Crab can retreat into its shell and 

 secure protection from predaceous fishes through the 

 anemone which, because of its toughness and stinging 

 cells, is avoided as food. It is not a long step from com- 

 mensalism to true parasitism. Parasites are organisms 

 that live temporarily or permanently in or upon other 

 organisms. They feed upon them or their food and be- 

 come more or less modified for this purpose. Among the 

 shore forms the parasites occur mostly among the worms, 

 particularly the flat-worms and the crustaceans. One 

 crustacean (Sacculina) attacks the Shore Crab (Carci- 

 nus) ; a similar form (Peltogaster) attacks the Hermit 

 Crab. The parasites in their adult form have degenerated 

 and bear no resemblance to crustaceans, and in both cases 

 have an important influence on the host. Many of the 

 parasitic flat-worms, especially the "flukes," pass certain 

 stages in their development in the mollusks dwelling along 

 the shore. Pearls are formed because of the irritation 

 set up by such parasites. 



Competition within the species. Not only is there 

 competition among the different species of animals at 

 the shore, but there is also competition within the 

 species itself. This is very well illustrated in the mus- 

 sel beds. The young of the species settle upon the 

 older forms and grow there. This may be continued 



