238 CLASSIFICATION OF PLANTS AND ANIMALS 



tionships in classification. For example, the wing of a bat, the 

 wing of a bird, the fore flipper of a dolphin, the fore leg of a lion, 

 and the arm of a man are homologous structures and show us 

 that these animals come in a rather general relationship. If we 

 look further, we find that all have another similar or homologous 

 structure called a vertebral column (the '^backbone")- Thus we 

 can at once classify these animals as vertebrates. 



Homology often aids us in understanding likenesses in structure 

 that are not easily found. It has taken a good deal of study to 

 show, for instance, that the paired fins of fishes are homologous 

 with the paired appendages of a dog or a cow. But by study- 

 ing the development of such structures, the relationship has been 

 made clear. 



On the other hand, we often find organs which do not have the 

 same structure or origin yet are used for similar purposes. Such 

 are the wings of a bird and the wings of a butterfly. Such struc- 

 tures are said to be analogous structures. Analogy is likeness in 

 function, regardless of origin. 



Classification of Animals. — We have found that a one-celled 

 animal can perform certain functions in a very simple way. Man 

 can perform these same functions in a very complicated way 

 because in him great division of labor is developed and extreme 

 complexity of structure is seen. Between these two extremes are 

 a great many groups of animals which can be arranged more or 

 less as a series. In the pages which follow, the great groups or 

 phyla of animals (Protozoa, Porifera, etc.) have been arranged 

 beginning with the simplest and ending with the most complex. 



I. Protozo'a (Gr., protos, first; zoon, animal). — Animals composed of a 

 cingle cell, reproducing by cell division. About 10,000 species are known. 



Amoeba Monosiga Vorticella 



Protozoa — all highly magnified. 



Plasmodia 



