28 



swimming or planktonic stages in the upper columns of 

 water. 



Any factor that might affect one of the units of 

 such an association, such as diminished food supplies or 

 other unfavorable physical or chemical conditions or 

 disease, might effectively upset a balance which could 

 bring about a change in the facies of the entire asso- 

 ciaticwio Such changes have doubtless occurred in the 

 past, and continue to occur in Santa Monica Bay, As 

 evidence, there are not only the dead remains of former 

 inhabitants, such as shell or tube fragments and bored 

 rocks, but there exist unusual abundances of some kinds 

 of animals which would not be normally expected unless 

 there was an unbalance of feeding types. These abundance 

 peaks are most prominent in Zone I (Limited Enriched 

 Sector)o It is noteworthy that areas of displaced popu- 

 lations are perhaps constantly being restocked by animals 

 of other kinds. Settling stocks as swimming or creeping 

 larvae may be constantly available from outside sources, 

 but only few kinds might be expected which can establish 

 themselves. The replacement of worn out, starved, or 

 overpolluted areas in Santa Monica Bay may be ecologically 

 complex. It may differ with time of year and with depth 

 and kind of bottom. The temperature, salinity, and pollu- 

 tion tolerances of the invading animals, are other 

 factors of importance. 



Animals can be further classified according to the 



