The Animal Kingdom 



THE PRINCIPLES of classification are es- 

 sentially the same for both plants and ani- 

 mals. At a very early date, however, animals 

 began to take a different path of evolution. 

 Consequently quite a different set of tax- 

 onomic criteria must be employed when one 

 seeks to classify the animals. 



SOME IMPORTANT CRITERIA 



Active locomotion is much more charac- 

 teristic of animals than of plants. Conse- 

 quently the organs of locomotion are apt to 

 be conspicuous among the distinctive fea- 

 tures of an animal. One-celled animals (Pro- 

 tozoa) are classified primarily on the basis of 

 their locomotor organs; and among multi- 

 cellular animals (collectively the Metazoa) 

 these organs often serve as important taxo- 

 nomic criteria. Moreover, the sensory-neuro- 

 muscular structures, which likewise are in- 

 strumental in the fulfillment of the food- 

 getting and other movements of the animal, 

 are apt to be distinctive in the different ani- 

 mal groups. 



But in seeking to ascertain the evolution- 

 ary status of an animal group, a wide variety 



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of questions must be considered. What is the 

 cellular organization of the group? Is it uni- 

 cellular, colonial, or truly multicellular? Is 

 the animal diploblastic or triploblastic? Are 

 there any special features that are distinc- 

 tive — in the embryo, or in the adult? How 

 far has the differentiation of special tissues 

 and organs progressed? What is the symmetry 

 of the animal? Is it spherical, radial, or bi- 

 lateral? What is the character of the digestive 

 system? Does it consist of gastric vacuoles; 

 or is it a saccular or tubular system? What is 

 the developmental status of the circulatory, 

 respiratory, excretory, and skeletal systems? 

 How did some higher animal groups man- 

 age to invade the land, leaving the age-old 

 aquatic environment of the ancestral species? 

 What is the pattern of the reproductive struc- 

 tures; and how have the reproductive proc- 

 esses become adapted to land conditions? 

 What is the importance of the group in na- 

 ture and in human affairs? And above all, 

 perhaps, what paleontological evidence is 

 available? And what does this evidence indi- 

 cate as to the origin, antiquity, and specific 

 relationships of each particular animal 

 group? 



