animal phyla

There are about thirty-something animal phyla that are recognized by invertebrate zoologists. There are two reasons the total number of phyla is imprecise. There is some subjectivity involved in determining whether a clade should be considered a single phylum or whether the group exhibits sufficient diversity to be divided into several. It's the taxonomic argument between the "splitters" and the "lumpers." In addition, new data can increase our understanding of evolutionary relationships. For example, based on new molecular evidence, the vestimentiferans are no longer considered a separate phylum, but are now recognized as a group of highly derived annelids.

There are quite a few phyla that are easily named by non-experts, even if they don't know the scientific names. The Arthropoda is the largest and includes the insects, crustaceans, centipedes, spiders, and scorpions. Echinodermata, Mollusca, and Annelida also include critters familiar to most, as do the Cnidaria and Porifera. There are also various other phyla of worms (flat, round, and others) that are speciose, although less well-known by those who haven't taken an invertebrate zoology class. However, in addition to these groups, which include many large-bodied representatives, there are also quite a few so-called "minor phyla" that contain only a few members. Even though they are not numerically dominant, these groups are important to our understanding of the evolution of organismal diversity. Dave Barry's take on the recently described phylum, Cycliophora, shows that minor phyla can appeal even to humorists.

Figuring out how many phyla there are and who belongs in which phylum is straightforward compared with the task of understanding the evolutionary relationships among phyla. Kenneth Halanych gives a very clear description of our current understanding of the phylogeny of invertebrate phyla. This paper was published in 2004, and there are already minor changes to the tree that I will discuss in future posts.