350 The Animal Kingdom 



Characteristics of the Phylum. — These animals forming the sub- 

 kingdom and phylum Protozoa are acellular or single-celled forms which 

 exist either individually or as members of a colony. If they are part 

 of a colony, they still retain their individuality. All members of the 

 colony are alike except those specialized for reproduction. 



In size they vary from 2 to 3 microns (one micron = 1/1,000 mm), 

 to several centimeters. Most, however, are less than 250 microns in 

 length. 



In symmetry, they vary from bilateral to radial, spherical, or asym- 

 metrical. Despite this variation, any one species ordinarily has a char- 

 acteristic, constant form. 



Locomotion is accomplished among the members of the various 

 classes by different means. It is by these varying locomotor apparatuses 

 that the classes are distinguished from one another. Some move by 

 pseudopodia or cytoplasmic protrusions ; others move by cilia or flagella 

 which are filamentous extensions ; and still others glide by movements 

 of the cell itself. The speed with which the animals move varies from 

 about 2 to 3 microns per second for those using pseudopodia to 2,000 

 microns per second among some of those using cilia. 



The internal organisation varies from group to group. Although 

 all species have at least one nucleus, some have several. Organelles are 

 present in most species. These, too, vary both in number and com- 

 plexity. 



Nutrition of the protozoans is of several types. Some forms pos- 

 sess green coloring matter and by photosynthesis manufacture their own 

 food. These are the holophytic species. Often these particular pro- 

 tozoans are classed as being intermediate in form between plants and 

 animals. The liolosoic forms are those which eat other plants or ani- 

 mals. Most protozoans may be included in this category. Some of the 

 parasitic forms can digest only that food which has already been 

 partially or fully dissolved. These are the saprosoic species. This lat- 

 ter group also includes interesting forms that can synthesize starches 

 and proteins from acids and simple salts. Usually any one species 

 utilizes only one method of nutrition, but occasional forms shift from 

 one type to another. 



In their mode of living the protozoans demonstrate a wide varia- 

 tion in both habit and habitat. A tremendous number of them are 

 free-living forms, while a great many others are truly parasitic. Be- 

 tween these two extremes there are various mutualistic associations 

 such as symbiosis and commensalism. In the symbiotic arrangement, 



