764 



ANIMALS AND THEIR ENVIRONMENT 



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DAYS 

 Figure 37.4. An experiment to demonstrate the competition between two closely 

 related species of paramecia which have identical niches. When grown separately in 

 controlled cultures with a fixed supply of food (bacteria), both ParaineciiDii caudatiim 

 and P. aurelia show normal S-shaped growth curves (solid lines). When grown 

 together, P. caiidatum is eliminated (dotted lines). (After Cause, from Allee, et al.: 

 Principles of Animal Ecology.) 



337. Commensalism 



Commensalism, the living together of two species, one of which 

 (the commensal) derives benefit from the association whereas the other is 

 unharmed by it, is especially common in the sea (Fig. 22.13£). Practically 

 every worm burrow and shellfish contains some uninvited guests that 

 take advantage of the shelter, and possibly of the abundant food, pro- 

 vided by the host organism but do it neither good nor harm. Certain 

 flatworms live attached to the gills of the horseshoe crab and get their 

 food from the scraps of the crab's meals. They obtain shelter and trans- 

 portation from the host but apparently do it no harm. Many oysters 

 and other bivalves have small crabs living in their mantle cavity, and 

 there is a species of small fish that lives in the posterior end of the 

 digestive tract of the sea cucumber! 



338. Protocooperation 



If both species gain from an association, but are able to survive 

 without it, the association is termed protocooperation. A number of 

 crabs put coelenterates of one sort or another on top of their shells, 



