THE INVERTEBRATE LEGIONS 87 



respiration, locomotion, and so on are carried out. It is true that some protozoans 

 are colonial— several living together as a single group— but none of these has a 

 division of labor among cells in which the cells are united into organs for 

 special purposes. 



This phylum is the most variable one in either the animal or plant kingdom. 

 No one body plan, form, or type of symmetry typifies all protozoans. Some 

 species, like Amoeba, are even able to change their form as they move about, 

 but most species have one form that typifies them throughout their lives. 

 Protozoans are among the largest of phyla, existing practically everywhere— in 

 sea water at all depths, in fresh water, on land, and broadcast through air as 

 spores. Some are parasitic, causing such diseases as amoebic dysentery, malaria, 

 and African sleeping sickness. 



Protozoans are all small. A microscope is required for seeing their details of 

 structure, though many can be seen with the naked eye. They range in size from 

 a few thousandths of a millimeter (1/25 of an inch is equivalent to 1 mm.) 

 to an extreme of over ¥i of an inch for some fossil, amoebalike protozoans with 

 skeletons. Protozoans reproduce by fission, a simple splitting of a cell in two, 

 a method which allows them to increase their numbers rapidly when the right 

 conditions prevail. Just when and where these conditions prevail is the key to 

 understanding protozoans, for they are all extremely sensitive to environmental 

 modifications. Under the proper conditions of salinitv, temperature, viscositv, 

 pressure, light, food, etc., the numbers of a certain species may become 

 incomprehensibly immense. If these conditions change only a little, the species 

 will often die off and produce dormant stages, such as resistant spores which 

 will become active again only when conditions once more become adequate. 

 But so great are the numbers of species of protozoans that almost any conditions 

 are adequate for at least one or two species. Rarely, if ever, are any waters 

 free of them. 



Protozoan behavior never varies much within a species. Some species are 

 light-sensitive, moving toward or away from it. All react to mechanical shock 

 and temperature change, and most can sense food, moving toward it. 



Most of the protozoans with which we shall be concerned are planktonic 

 and occupy a vital place at the very base of the food chain, either deriving their 

 body-building energy directly from the sun or from bacteria or other minute 

 particles which they eat. 



Flagellates: Class Flagellata or Mastigophora — Figure 23 



This group, which forms a link between true animals and true plants (algae), 

 occupies an extremely important place in evolution. Some of them contain 

 chlorophyll and are photosynthetic, looking very much like some single-celled 

 green algae (Dunaliella, fig. 19'). Others eat food such as bacteria or decayed 

 substances as do animals. The photosynthetic species have a light-sensitive eve 

 and move toward light. They are extremely important in the sea as basic 

 converters of light energy to carbohydrates. 



All of the flagellates have one or several flagella, or whiplike filaments, which 

 move the animal. Some flagellates are colonial with several cells living together 

 as a unit, and these bear a remarkable similarity to the sponges in that there is a 



