132 



KINGDOMS MONERA AND PROTISTA: 



consider them as such, for many reasons. For ex- 

 ample, certain bacteria and the sUme molds appear 

 very closely related, so close that even experts can be 

 perplexed by their true relationships. In addition, it 

 was already mentioned that the slime molds are al- 

 lied to the Sarcodina. However, they are treated as 

 separate kingdoms in this book because it seems con- 

 venient to stress organizational differences that im- 

 ply two major steps in evolution. Once Protista is 

 considered as a kingdom, the six phyla assigned to it 

 must be included, since they most likely represent a 

 closely knit phylogenetic group. 



If inclusion in a taxonomic system of a separate 

 Kingdom Protista tends to simplify the understand- 

 ing of organism evolution, why is this taxon often 

 avoided in textbooks and elsewhere? There is no sim- 

 ple answer to this question. Perhaps the answer is 

 that too little is known of the fossil record of protis- 

 tans. The phylogeny of the group is based largely 

 upon conjecture about the implications of various 

 structures in living creatures. Unfortunately, this 

 may not provide a true picture of protistan phylog- 

 eny; analyses of structures in living organisms, 

 no matter how detailed, do not reveal a single pos- 

 sible path for their past evolution. Present features 

 often contradict one another in any interpretation of 

 their evolution. Such contradiction is understand- 

 able because only certain structures reflect evolution- 

 ary trends, whereas others portray adaptations to en- 

 vironments. In other words, similar features in 

 protistans can be due to common evolutionary his- 

 tory or to a common environmental history. In like 

 habitats, chance hereditary changes and selection of 

 the changes would be likely and would lead to sim- 

 ilar-appearing creatures. Moreover, it is very 

 difficuh, if not impossible, to be sure whether any 

 particular similarity is due to common environment 

 or to common ancestry. This is the case with simi- 

 larities among any creatures where a fossil record is 

 poor or absent. 



SUBKINGDOM PROTOZOA 

 (Protozoans) 



Although frequently called simple and primitive, 

 living members of the protozoans have evolved sep- 

 arately through eons of time to their present form. 

 Their simplicity and primitiveness exist only in their 

 physical resemblance to the most archaic protists. 



Living protozoans include the flagellates, which 

 often are believed to approximate the ancestors of all 

 other protozoans and of members of the plant and 

 animal kingdoms. It is thought that archaic flagel- 

 lates gave rise to most living protozoans, to sponges 

 (Porifera), and to green algae (Chlorophyta) via 

 innumerable ancestral forms. Also, in one hypothesis 

 of the origin of the animal phyla Mesozoa and 

 Coelenterata, the flagellates are believed to have been 

 ancestors. However, both coelenterates and meso- 

 zoans could have originated otherwise. 



Diagnosis: body cell-like but the single proto- 

 plasmic mass is an organism; hence, protistans are 

 structurally and functionally somewhat unlike true 

 plant or true animal cells; nucleus differentiated, 

 complete and complex, unlike the simpler struc- 

 tured Monera counterpart. 



Structure: symmetry of all types; small, solitary or 

 colonial creatures; most are microscopic, but a few 

 are visible to the naked eye; occur in most land, fresh- 

 water, and marine habitats as sessile, or free-living 

 forms; many aquatic forms contribute to plankton; 

 many are internal or external parasites of plants, 

 animals, and other protistans; cell division regularly 

 by mitosis or meiosis, amitosis uncommon or absent. 



Protozoans are assumed to be acellular by many 

 biologists because the "cells" of these creatures are 

 the organism and often contain certain complex struc- 

 tures that are not found in the cells of multicellular 

 life. Many of these complex protozoan structures 

 have functions that are similar to the organs of ani- 

 mals. For this reason, the complex structures are 

 called organelles, literally "little organs." 



Many other biologists would not consider proto- 

 zoans acellular; rather, they would call them single- 

 celled animals that occur either singly or colonially. 

 Their reasoning is that cell is a structural designation, 

 ind even if specialized, the protozoans possess the 

 structures which fit the name "cell." (In fact, many 

 biologists assume monerans are cells.) Moreover, 

 some cells in multicellular organisms display great 

 complexity. However, even this group of biologists 

 agrees that there is a functional distinction between 

 the protozoan structure and the cell of a plant or ani- 

 mal. The protozoan is functionally an organism; 

 generally plants and animals have unlike cells for 

 unlike functions. 



The disagreement among biologists need not be 



