Classifications

In the Five Kingdom system, the domain Eukaryota is classified into the following kingdoms:

Monera: This kingdom includes all prokaryotic organisms, such as bacteria and cyanobacteria.
Fungi: This kingdom includes fungi such as mushrooms, molds, and yeasts.
Plantae: This kingdom includes all multicellular plants.
Protista: This kingdom includes a diverse group of mostly unicellular organisms, such as algae, protozoa, and slime molds.
Animalia: This kingdom includes all multicellular animals.

In the Eight Kingdom system, the classification is further refined. Monera is divided into: Archaeobacteria and Bacteria. Protista and Plantae are further divided into: Archezoa, Chromista, Protista, and Plantae.

Archaeobacteria (Archaea)
Bacteria (Eubacteria)
Fungi (Mushrooms, Molds, and Yeasts)
Archezoa (Primitive Eukaryotes)
Chromista (Algae, Diatoms, and other similar organisms)
Protista (Diverse group including some Algae and Protozoa)
Plantae (Multicellular Plants)
Animalia (Multicellular Animals)

Classification according to tissue types

The development and evolution of true tissues have significant implications for the complexity and specialization of organisms.

Sponges (Phylum Porifera) are simple organisms that consist of a mass of cells without true tissues or organs. They are often referred to as Parazoa, indicating their primitive level of organization. In contrast, organisms with true tissues and organized structures are known as Eumetazoa.

Classification according to dermal layers

Germ layers are groups of cells in the embryo that interact during development to form the tissues and organs of the animal.

Ectoderm: The outermost layer, which gives rise to the epidermis and nervous system.
Mesoderm: The middle layer, which forms muscles, the circulatory system, and other internal structures.
Endoderm: The innermost layer, which forms the lining of the gut and associated structures.

Symmetry and Cephalization

The evolution from radial to bilateral symmetry marks a significant transition in the complexity of body plans. Bilateral symmetry is associated with cephalization—the concentration of sensory organs and nerve cells at the anterior end (head) of the animal.

Among Eumetazoa, those with radial symmetry, such as members of the phylum Cnidaria (including sea anemones and jellyfish), are classified as Radiata. This group is characterized by their body plan, which allows for symmetry around a central axis.

The comb jellies (Phylum Ctenophora) are, despite their jelly-like appearance, exhibit bilateral symmetry, meaning their body plan is divisible into symmetrical halves on either side of a unique plane.

Classification according to body cavity development stages

Platyhelminthes (planarians, flukes, and tapeworms) and Nemertinea (proboscis worms) are part of the Bilateria, a group of animals with bilateral symmetry. Both Platyhelminthes and Nemertinea are acoelomates, meaning they lack a coelom, which is a body cavity lined by mesoderm. Platyhelminthes has head and simple gastrovascular cavity. Nemertinea has complete digestive tract with simple vascular system, and hemoglobin.

Both Rotifera (wheel animals) and Nematoda (roundworms) are pseudocoelomates. They have a pseudocoelom, which is a body cavity that is not fully lined by mesoderm. Rotifera(wheel animals) and Nematoda are pseudocoelom which have a complete digestive tract and vascular system.

Bilaterally symmetrical animals with a true coelom are referred to as coelomates. A coelom is a fluid-filled body cavity completely lined by mesoderm.

Acoelomates: Bilaterians without a body cavity (e.g., Platyhelminthes, Nemertinea).
Pseudocoelomates: Bilaterians with a partially lined body cavity (e.g., Rotifera, Nematoda).
Coelomates: Bilaterians with a fully lined body cavity (e.g., annelids, mollusks, arthropods, echinoderms, vertebrates).

Among the coelomates, those that develop determinate cells (cells with predetermined fates) are called protostomes. In these organisms, the coelom forms through a process called schizocoely, where the coelom splits from the mesoderm. Protostomes undergo spiral cleavage, a type of embryonic development characterized by the spiral arrangement of cells.

Deuterostomes vs. Protostomes

Protostomes undergo spiral cleavage, a type of cell division where embryonic cells divide at oblique angles, producing a twisted arrangement. This cleavage is also determinate, meaning that the fate of each cell is fixed early in development; if a cell is removed, the embryo cannot compensate. The coelom in protostomes forms through schizocoely, where mesodermal tissue splits to create the body cavity. Importantly, in protostomes, the blastopore—the first opening formed during gastrulation—develops into the mouth. This lineage includes diverse phyla such as arthropods, mollusks, and annelids, all of which share these embryological traits despite their morphological diversity.

In contrast, deuterostomes exhibit radial cleavage, where cells divide parallel or perpendicular to the axis, resulting in a more symmetrical arrangement. Their cleavage is indeterminate, meaning that early embryonic cells retain the potential to develop into complete organisms—an attribute that allows for greater developmental flexibility and is the basis for identical twinning in some species. The coelom in deuterostomes forms via enterocoely, where it buds off from the archenteron (primitive gut). In this group, the blastopore becomes the anus, with the mouth forming secondarily. Deuterostomes include phyla such as echinoderms and chordates, the latter encompassing all vertebrates.