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Phylum Definition

Phylum is a taxonomic ranking that comes third in the hierarchy of classification, after domain and kingdom. Organisms in a phylum share a set a characteristics that distinguishes them from organisms in another phylum. The qualities that group animals into a phylum have changed throughout scientific history, as better methods have arisen to determine how groups of animals are related. Modern phylogenic systematics, or simply phylogenetics, uses a variety of traits to map the changes between different groups of animals. Different hypotheses of how a phylum is related to other phyla (plural of phylum) create different cladograms, which display the relationships visually. The cladogram that is the most parsimonious, or has the fewest number of changes, is usually accepted as the most likely hypothesis of the relationships between different phyla. Plants, protists, and bacteria have also been categorized into divisions historically. The word was later changed to phylum.

There are approximately 35 animal phyla, 12 plant phyla, and 7 phyla of fungi. The bacteria, including the archaea, are grouped into roughly 34 phyla, although the relationships between these groups are not as well established. The exact numbers of phyla are never known for sure, as new evidence and techniques are discovered. For instance, with the advent of genetic testing many groups that were thought to be monophyletic were found to have very different genetics. Monophyletic is a term that describes a complete group, with all the common ancestors. Polyphyletic groups contain organisms that do not share a recent ancestor, and many more groups would have to be included to make the phylum monophyletic. In these cases, the phylum is split into multiple phyla. Other times, two different phyla are found to be closely related and will be combined into the same phylum.

Examples of Phylum

Phylum Chordata

Think of an animal that lives in the zoo. Unless you thought of an octopus or one of the insects in the rainforest pavilion, chances are you thought of an animal in the phylum Chordata. This phylum includes all animals with a notochord, a dorsal hollow nerve chord, and a muscular postanal tail. These three features are seen in every member of this phylum, even humans. Like humans, many animals lose some of these features as they develop. This phylum includes everything from the smallest fish and tunicates, to the great apes. It is one of 88 different known phyla. It terms of species, it only has around 50,000 described species. The other phyla combined have well over 2 million described species, with the phylum Arthropoda having well over a million described species itself.

Phylum Arthropoda

By far the largest individual phylum, the Arthropoda includes all the segmented animals will hard skeletons that we know as insects and crustaceans. Everything from deep sea crabs to mosquitoes fit in this phylum. The diversity of the arthropods comes from their ability to adapt their hard exoskeleton into almost any shape or function. Because of this, the arthropods have adapted to survive in every environment on the planet.

Phylum Mollusca

This phylum is where the octopus and its relatives are grouped together. The mollusks include all the snails, slugs, clams, mussels, squids and other soft-bodied animals. They often secrete a shell to protect their soft bodies. As the individuals die over time, their shells are shed to the bottom, where they accumulate. Waves push the shells and coral matter onto the beach, where it is repeatedly crushed by the waves. In this way, mollusk shells contribute a large majority of the sand that exists on the best beaches. While most of the members are relatively unintelligent and rely on sessile filter feeding, many species are very intelligent, competing with organisms like dolphins and pigs. Different species of octopus, for instance, have completed highly complex tasks to obtain food. Some squid hunt in large predatory packs which work cooperatively to bring down large prey.

  • Kingdom – The classification group above phyla, which includes organisms of a certain type, like animals.
  • Class – The ranking below phyla, whose members share the characters of the phylum, but are have advanced characters that make them unique.
  • Classification – The process of organizing the life on Earth into a meaningful framework for understanding the relationships between animals.
  • Phylogenetics – Comparing the relationship between animals by comparing many of their traits at once.


1. Which group of organisms is most closely related?
1. Phylum Cnidaria, Class Hydrozoa
2. Phylum Arthropoda, Class Insecta
3. Phylum Cnidaria, Class Cubozoa

A. 1 and 2
B. 1 and 3
C. 2 and 3

Answer to Question #1
B is correct. Groups 1 and 3 are the most closely related, because they both belong to the phylum Cnidaria. Without knowing anything about these groups of organisms, simply knowing the phylum they belong to tells you the answer. Answers A and C both contained organisms that belonged to different phyla. If a group is not in the same phylum as another group, the two are less related than organisms in the same phylum. If you didn’t know, phylum Cnidaria includes all animals with stinging cells known as nematocysts. Hydrozoa and Cubozoa are two different classes of jellyfish-like organisms. The Cubozoans contain the box jellyfish, one of the most poisonous animals in the world. Phylum Arthropoda, Class Insecta are the insects.

2. Some scientists argue against the use of the phylum as a method of classification. Which of the following could be an argument why the phylum is not a good unit to use?
A. The group is too diverse
B. The relationships between the organisms are too convoluted
C. Organisms can lose the characteristics that would otherwise classify them within a phylum

Answer to Question #2
C is correct. Until scientist started practicing embryology to view the development of individual organisms, it was not known that even humans retained some of the basic characteristics of their phylum. While our postanal tail and gills eventually disappear, it was much easier to classify the phylum Chordata, knowing that its members must have the traits at some point in development. Some scientists say this is a downfall of this classification method, because we could never know if we were accidentally excluding an organisms just because we hadn’t studied it enough to know whether it had a trait. These scientist argue for a more cladistics approach, where organisms are mapped rather than grouped, to emphasize the continual evolution of life and changing of the groups.

3. The phylum Echinodermata contains starfish and their relatives. Which of the following statements contains a shared characteristic of the phylum Echinodermata?
A. Brittle stars, unlike other starfish, filter feed from the environment.
B. The sea urchins and sand dollars are closely related to starfish, but secrete a hard shell for protection.
C. The starfish and all their relatives possess pentamerous radial symmetry.

Answer to Question #3
C is correct. The only statement that describes a characteristic that applies to all echinoderms is answer C. Pentamerous radial symmetry simply means groups of five arranged in a circle. However, it was not necessary to understand any of the technical jargon to answer this question. A shared characteristic is one that all members of the phylum contain. Answers A and B both referred to special adaptations of members of the phylum Echinodermata. Not all members contain these traits, therefor they are not considered shared ancestral characters of the phylum.

Cite This Article

MLAAPAChicago Editors. "Phylum." Biology Dictionary,, 31 Jan. 2017, Editors. (2017, January 31). Phylum. Retrieved from Editors. "Phylum." Biology Dictionary., January 31, 2017.

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