THE CLASSIFICATION OF ANIMALS 



purpose the functions of the parts of an organism are not important, for even 

 in animals of the same phylum comparable parts often have entirely different 

 functions. The basic structure of the parts is significant because this struc- 

 ture is found to be the same despite modifications correlated with difTerent 

 uses in different animals. The fore limbs of various vertebrate animals 

 furnish examples of modifications of comparable parts in adaptation to diverse 

 functions. The wings of birds and of bats, the fore limbs of horses, dogs, mice, 

 frogs, monkeys, and so on, have all been modified in various directions in 

 adaptation to various uses; but the underlying structural basis of the fore 



Chordata 



Echinodermata 



Hemichordata 



Mollusca 



Brachiopoda Dipleurula? 



Chaetognatha 

 ENTEROCOELA SCHIZOCOELA 



PSEUDOCOELOMATA 



EUCOELOMATA ^ 



ACOELOMATA 



PRIMITIVE ACOEL? 



Arthropoda 



Onychophora 

 Annelida 



Echiuroidea 



Aschelminthes 

 Acanthocephala 

 Nemertinea 

 Platyhelminthes 



BILATERIA 



-Ctenophora 

 -Coelenterata 



RADIATA 



PLANULA-LIKE FORM? 



EUMETAZOA 



Flagellata 



PARAZOA (Porifera) 



MESOZOA 



Other Protozoa 



Fig. 7.3. A phylogenetic tree of the Animal Kingdom, designed to show the possible interre- 

 lationships of the various phyla. Such a figure should be interpreted as a statement of evolu- 

 tionary probabilities and not as a precise representation of the exact course of evolutionary 

 change. 



219 



