Members of the microbial kingdom Protista originally were defined by structure (mainly unicellular eukaryotes) and by the difficulty to classify them as either plant, fungi or animal.  More recently, the concept of protists was expanded to include certain multicellular organisms such as kelp (Copeland, 1956). Thus defined, members of Protoctista range from microscopic one-celled organisms like dinoflagellates, to multicellular organisms, like seaweed. To untangle this confusing kingdom, biologists now are turning to molecular analysis. 

When following the traditional five- or six-kingdom classification, the Protist group contains all eukaryotes that are not fungi, plants or animals. There are unicellular, colonial, and multicellular forms, some of which show cell specialization.  Protists groups include both autotrophs and heterotrophs, some of which function as detrivores.

Animal-like groups are often referred to as Protozoans.  The term Protozoa dates back to when members of this group were considered "first animals."  Plant-like forms are generally called algae.

Traits such as method of motility, presence or absence of a shell, manner of obtaining nutrition, and reproducing, are used to categorize and discuss this diverse group, but it is important to remember that these traits do not necessarily reflect evolutionary history.  Recent work suggests that green and red algae are more closely allied with land plants, and that slime molds are more closely allied to animals (Baldauf, et al. 2000).

The best choice of optics depends on the specimen to be viewed. Specialized optics can give wonderfully detailed views of objects that are disappointing in bright field, but bright field often is a better choice. Bright field optics are usually-but not always-the best choice for viewing stained tissue.
    
A naturally pigmented specimen, such as Spirogyra may appear more dramatic with dark field or D.I.C. optics, but the cell divisions and chloroplasts are distinct in bright field, and the colors are true to nature. Phase contrast does not contribute additional information, and the halo that typically surrounds a specimen actually detracts from ideal contrast. On the other hand, phase contrast optics give the best view of spore-forming bacteria such as Bacillus thuringiensis. Dark field optics show the cell walls and spores with excellent resolution. To see such features in bright field, one must stop down the condenser aperture, causing distortion of the details.
 
Differential interference contrast does not do much for our views of a specimen such as Spirogyra or Bacillus. On the other hand, D.I.C. optics increase the depth of focus, making features of an object such as an amoeba very distinct even if they do overlap each other.