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.
Classification according to body plan
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.
In contrast, deuterostomes exhibit indeterminate cleavage, where the fate of each embryonic cell is not predetermined early on. This allows for more flexibility in development. Deuterostomes undergo radial cleavage, where the cells are arranged in a radial pattern. The coelom in deuterostomes forms through enterocoely, where the coelom buds off from the gut.