A worm is an animal. While the word “worm” is a common, non-scientific term used to describe many different organisms that share a long, soft, legless body shape, all organisms referred to as worms fall squarely within the Kingdom Animalia. Understanding the rules of the animal kingdom provides the scientific context for why these creatures are classified alongside mammals and birds.
What Defines the Animal Kingdom?
The classification of any organism into the Kingdom Animalia is based on a specific set of biological criteria that distinguish animals from plants, fungi, and single-celled organisms. All animals are multicellular, meaning their bodies are composed of numerous cells working together. These cells are also eukaryotic, containing a nucleus and other membrane-bound organelles.
Animals are heterotrophic, meaning they cannot produce their own food through processes like photosynthesis. Instead, they obtain energy and nutrients by ingesting organic material from other organisms. Furthermore, unlike plants and fungi, animal cells entirely lack a rigid cell wall, which allows for flexibility and movement.
Classification as Invertebrates
Once an organism meets the criteria for the Animal Kingdom, scientists narrow its classification based on structural features. Worms belong to the major group known as invertebrates, a category that includes over 90% of all animal species.
An invertebrate is any animal that does not possess a vertebral column, or backbone. This absence distinguishes them from vertebrates, such as fish, amphibians, reptiles, birds, and mammals. Worms are soft-bodied animals that lack this internal bony structure, placing them alongside insects, spiders, and mollusks.
The Three Major Types of Worms
The common term “worm” is used to describe animals from several different phyla, representing distinct evolutionary paths and body structures. The three main phyla commonly referred to as worms are Annelida, Nematoda, and Platyhelminthes. Each phylum possesses a unique body plan that reflects its biological complexity.
Phylum Annelida (Segmented Worms)
This phylum includes segmented worms, with the earthworm serving as a familiar example. Annelids possess bodies divided into a series of repeating segments or rings. This segmentation allows for more specialized body systems and efficient movement through soil or water.
Phylum Nematoda (Roundworms)
Nematoda is made up of the roundworms, which include parasitic species like hookworms. These worms are characterized by a cylindrical, unsegmented body that is tapered at both ends, giving them a round appearance in cross-section. Nematodes are incredibly abundant and often microscopic, inhabiting nearly every environment on the planet.
Phylum Platyhelminthes (Flatworms)
This phylum consists of the flatworms, including organisms such as the common planarian and parasitic tapeworms. Flatworms are defined by their dorsoventrally flattened bodies, meaning they are thin and ribbon-like. They are the least complex of the three groups, typically lacking advanced circulatory or respiratory systems.