ABSTRACT
The animal kingdom includes between thirty-one and thirty-six phyla, depending on the taxonomist. This uncertainty reflects a lack of knowledge about the phylogenetic relationships of some of the groups. The term “phylum” applies to a monophyletic group of animals with a distinctive Bauplan expressed in many variations. For example, there is a distinctive body plan for the mollusks, despite the phenotypic differences among snails, oysters, and octopuses. One major problem is that the monophyly of certain groups is not yet firmly established (Willmer, 1990). For example, some taxonomists argued that arthropods should be reassigned to more than one phylum and have recognized up to three different phyla. Similarly, whereas the monophyly of animals is widely supported, some
taxonomists argue that sponges, cnidarians, and bilaterals could have evolved from different protist ancestors.
