Living organisms are generally classified as uni- 

 cellular (the individual is a single cell) 

 or multicellular (the individual is an or- 

 ganized collection of specialized cells). 

 Although such categories imply rigid dis- 

 tinctions, we must remember that dis- 

 crete boundaries do not exist in nature 

 but only in the minds of biologists. In 

 reality, there is a middle ground oc- 

 cupied by organisms that are neither 

 wholly unicellular nor wholly multicellu- 

 lar. For example, many protozoa, algae, 

 fungi, and bacteria are transiently colo- 

 nial; they come together for a time as 

 loose clusters of quasi-independent cells 

 with no organization or speciahzation. 

 Others attain a primitive level of organ- 

 ization in which some cells of the colony 

 become specialized for feeding, others 

 for sexual reproduction, etc. A highly 

 complicated level is attained by one such 

 BeainninaS group, the cellular slime molds. During 



growth, the cells exist as orthodox uni- 

 Of cellular forms, each living independently 



-- I , , III of its neighbors. After growth has ceased, 



iVIlllTlceillliar ^.j^g ^^n^ come together and cooperate to 



Oraanization produce a single, organized, multicellular 



structure with well-differentiated tissues. 

 Many algae, fungi, and protozoa, 

 and also one group of bacteria, are per- 

 manently colonial, because the cells do 

 not separate after reproduction but in- 

 stead adhere to one another. In some 

 cases these become merely tangled amor- 

 phous masses of cells and we can see 

 nothing except the cells that might be 

 called individuals. In others a great deal 

 of organization is attained. The individ- 



The 



