242 ^"^ ANIMAL KINGDOM 



Arthropoda 

 Onychophora 



Annelida 

 Mollusca 



Other 



Eucoelomata 

 Nemertea — / ^->Aschelminthes 



Platyhelminthes 



Porifera 



► Chordata 



Hemichordata 



Echinodermata 



Entoprocta 



Coelenterata 

 Ctenophora 



Protozoa 

 Figure 13.3. A suggested phylogeny of the Metazoa, showing all of the major phyla 

 and some of the minor phyla. The Entoprocta and other minor phyla will be discussed in 

 Chapter 18. 



Enterocoelomata respectively. It will be clear from information that will 

 be presented in the following chapters that these are distinct groups. 

 Between the chapters that deal with these two series lies a chapter (18) 

 on minor phyla, some of which can easily be allied with one of the 

 series and some of which cannot. At this introductory level it will be 

 sufficient to recognize that there are two major groups of eucoelomates 

 and that a number of smaller phyla are left over. 



This discussion of possible evolutionary relationships among phyla 

 can be summarized in a phylogenetic tree (Fig. 13.3). This is, of course, 

 nothing more than a guess based on the evidence now available. A 

 variety of other schemes are just as plausible. A common variant is one 

 in which the two series of eucoelomates are derived separately from a 

 flatworm stock. 



Questions 



1. In what ways do sponges differ from all other metazoa? 



2. List the characteristics of flatworms that are not found in coelenterates. 



3. Compare the pseudocoelom and the eucoelom. 



