In biology, a phylum (division when referring to plants) is a taxonomic ranking that consists of organisms that share a unique set of characteristics. For plants and animals, as well as other organisms that do not fall into the two categories, the ranking (phylum) generally comes third in the hierarchy with the first two being domain and kingdom.
Whereas a kingdom consist of organisms that share a common origin/ancestry over the course of evolution (Plantae, Protista, Monera, Fungi, and Animalia), the phylum divided the kingdom into multiple groups based on given characteristics.
While recent phylogenetic use a variety of traits to identify the main differences between organisms for better classification, the basic body plan/organization has proved to be one of the most effective strategies/approaches for grouping organisms within the appropriate phyla (singular phylum).
* A phylum is further divided into several classes.
* While members of a given phylum may have a number of morphological/structural differences, they tend to share a specific structural characteristic.
In order to get a better understanding of phyla, it's worth looking at several plants and animals and how they are grouped:
As the name suggests, vertebrates are animals that have a backbone or spinal (vertebral) column which surrounds the primary nerve cord (spinal cord). All animals with a vertebral column belong to the phylum Chordata.
Some members of this group include:
Fish - Like other vertebrates, fish have a backbone (known as the spine). Each vertebra consists of a neural and hemal spine, neural and hemal arch, as well as a centrum. There are also two sets of ribs (ventral and dorsal ribs) connected to the vertebrae. However, they do not have a sternum. At the caudal region, the vertebrae serve an important role in protecting the caudal artery and vein. The entire skeletal system, on the other hand, protects various body organs.
* Some fish (e.g. rays and sharks etc.) are known as cartilaginous fish because their skeletal system largely consists of cartilage instead of pure bone.
Mammals - Essentially, mammals are warm-blooded animals that tend to nurse their young. They are some of the most common vertebrates on land. However, they can also be found in aquatic environments, e.g. dolphins.
Despite their many differences (e.g. different number of legs or no legs, size, etc.), all mammals have a spinal column. More interestingly, studies have shown that despite the significant variation in size, e.g. from mouse to giraffe, the majority of mammals have seven (7) cervical vertebrae and between 19 and 20 thoracic and lumbar vertebrae.
Ribs connected to the vertebrae are also connected to the sternum in mammals. Here, they are involved in a number of functions including protecting various internal organs within the thoracic cavity, respiration, and providing support to the trunk.
Birds - Generally, birds are characterized by the presence of feathers, wings, a beak, and the fact that they lay eggs. Compared to mammals, birds have more cervical vertebrae (between 12 and 25 depending on the type of bird). This is particularly important for birds because it allows the neck to be more flexible.
As well, the vertebral column in the lumbar and thoracic region is less flexible which protects the body against the powerful force generated during flight. The rib originating from the vertebrae are again connected at the keel-shaped sternum.
Amphibians - Unlike mammals, amphibians are cold-blood animals that start their lives in water and later move to land. In frogs, the column consists of 10 vertebrae and connects to the skull through a single cervical vertebra (atlas).
Reptiles - Reptiles are also cold-blooded animals (they also lay eggs). The vertebrae consist of the cervical vertebrae, the trunk, sacral, and caudal vertebrae.
Unlike vertebrates, invertebrates are cold-blooded animals that do not have a backbone. They can be found in various habitats in both terrestrial and aquatic environments.
While they all lack a backbone (vertebral column), they have many differences and are therefore divided into several phyla that include:
Phylum Porifera - The Phylum Porifera consists of organisms known as sponges. As the name suggests, these organisms bear many pores on their surface. They also lack complex organs found in many other animals (heart, organs of the digestive system, etc.)
Phylum Cnidaria - Members of this phylum are jelly-like animals and have tentacles. They are commonly found in aquatic environments where they may be attached to a substratum or freely swimming. Some members of this group also possess stinging cells (nematocysts) used for defense.
Phylum Platyhelminthes - Also known as flatworms, members of the Phylum Platyhelminthes worm-like animals characterized by flattened bodies. Though members of this group have a number of organs (e.g. reproductive organs and sensory organs etc.), they do not have a circulatory system and anus.
Phylum Echinodermata - Echinoderms include starfish among other animals characterized by a spiny surface, coelom, water vascular system and endoskeleton.
Phylum Mollusca - Mollusks are found in aquatic and terrestrial environments are characterized by a soft body (some have a hard shell protecting the body). The body of these organisms (the majority of mollusks) consists of several key features that include a muscular foot, mantle, and the visceral mass.
Phylum Nematoda - Nematodes are worm-like animals characterized by an elongated, slender body. Though they vary in size, they are all bilaterally symmetrical with tapered ends.
Phylum Arthropoda - Arthropods are characterized by an exoskeleton and jointed limbs
Phylum Annelida - Annelids are worm-like organisms (leeches, earthworms, etc.). They are characterized by an elongated muscular body that is segmented.
Like animals, plants are divided into several phyla depending on given structural/morphological traits.
Bryophyta - Because they lack a true vascular system, members of the phylum Bryophyta are small in size and are commonly found in moist and shaded areas. They are also characterized by leaf-like structures, a complex stem region known as hydroids, as well as root-like structures known as rhizoids.
Pteridophyta (ferns) - Members of the phylum Pteridophyta are also simple plants. While they have a stem (Rhizome), leaves, and roots, they do not produce flowers or seeds
Coniferophyta - Like many other plants, members of the phylum Coniferophyta (Gymnosperms) are characterized by roots, leaves, branches, and the stem. However, they do not produce fruits and flowers. For this reason, the seeds are mostly uncovered (naked). This is one of the main characteristics of these plants.
Anthophyta (angiosperms) - Also known as flowering plants, Angiosperms are perhaps some of the most common types of plants throughout the world. In addition to the typical parts associated with plants (stem, leaves, roots, etc.), they produce flowers, and seeds are contained in the fruits.
Marchantiophyta - Like mosses plants, liverworts do not have a true vascular system. They are also characterized by a thallus (leaf-like structure) and rhizoids that attach the plant to substrates.
Anthocerotophyta - Members of this phylum are known as hornworts. Like mosses plants, hornworts are small plants that lack a true vascular system. They are also characterized by a leaf-like thallus and root-like structures known as rhizoids.
Looking at all the phyla highlighted above, it becomes evident that they share one or several characteristics (E.g. all chordates have a vertebral column). Although this mode of classification, based on various physical traits, has been used for a while, it's worth noting that it has faced opposition for several reasons.
One of the criticisms often presented is that some ancestral traits associated with some organisms may have been lost by others. Here, the criticism suggests that using the body plan approach fails because it's possible that over time, some animals may have lost traits that would have otherwise allowed them to be appropriately classified in the right phylum.
Based on this criticism, it's possible that some organisms are inappropriately classified. And so, this approach is viewed as being more appropriate for classifying current or recently extinct organisms.
Based on the genetic relationship between various organisms, researchers have been able to reunite a number of organisms (place them in the same group) that were once placed in different groups. For instance, while bearded worms were once thought to be a unique group, molecular studies in the 21st century found that they belong to the phylum Annelida.
A good number of experts have continued to promote this approach for taxonomic classification. However, as is the case with the body-plan approach which considers physical characteristics, this approach has faced a number of challenges over the years. As a result, there have been those who feel that the phylum concept as a whole is outdated and should be abandoned altogether.
Elizabeth C. Davis-Berg. (2011). Teaching the Major Invertebrate Phyla in One Laboratory Session
Fabio Galbusera and Tito Bassani. (2019). The Spine: A Strong, Stable, and Flexible Structure with Biomimetics Potential.
Jeanette Wyneken. Skeletal Anatomy and Function In Reptiles.