Classification is essential to studying animals (and other organisms). It is necessary for tracing the evolutionary relationship of all organisms. The ordering of the taxa can be done in various ways. The closely related organisms are put together, forming a taxon. There are fundamental features common to various individuals based on which they are grouped. Some of the features used for classification are as follows.
Classification by body organization
- Cellular organization: Specialized cells like choanocytes and amoebocytes are present but lack proper tissues. Example- Porifera
- Tissue level of organization: Specialized cells are organized into tissues; however, tissues are not aggregated to form organs. Example- Cnidaria (=Coelenterata)
- Organ level of organization: Tissues are organized into proper organs, with a solid mass of mesodermal parenchyma cells surrounding the organs. Example- Platyhelminthes
- Organ-system level of organization: Many organs work together and form an organ system. Example- Members of Aschelminthes and all other higher phyla.
Organisms having organ system levels of the organization may have different types of organ systems. Like the digestive system may be:
- Incomplete: Only one opening serving both as mouth and anus. Example: Platyhelminthes.
- Complete: Separate openings serving as mouth and anus.
Similarly, the circulatory system may be:
- Open type: Organs are directly bathed in the blood and lack blood vessels. Example: Arthropoda and members of Order Echiuroidea of phylum Annelida.
- Closed type: Blood flows through a closed system of blood vessels.
Classification by body symmetry
- Asymmetrical: When there is no plane of symmetry, the organism is called asymmetrical. Example: Poriferans
- Radially symmetrical: Organisms that can be divided into two halves by sectioning through any plane passing through the centre, I.e., they have more than one plane of symmetry. Examples: Cnidaria, Ctenophores & Echinoderms (the echinoderms exhibit bilateral symmetry in the larval stage)
- Bilaterally symmetrical: Organisms possessing the right and left sides that approximately mirror each other’s images (at least superficially). It is correlated with cephalization, the concentration of nerve and sensory organs at one end resulting in distinct anterior and posterior ends. There is only one plane of symmetry passing through the anteroposterior axis. Examples- Platyhelminthes, Aschelminthes, and all other higher phyla.
Classification by number of germ layers
Multicellular organisms are classified based on the number of germ layers (groups of cells that behave as a unit during the early stages of embryonic development and give rise to different tissue and organ systems in adults). Organisms are divided into two groups based on the number of germ layers.
- Diploblastic: Organisms with two germ layers, outer ectoderm and inner endoderm (gastrodermis). An elastic support system is a thin, gelatinous layer between the ectoderm and endoderm of diploblastic organisms called mesoglea. Examples: Cnidaria and Ctenophora.
- Triploblastic: Organisms with three germ layers, outer ectoderm, middle mesoderm, and inner endoderm. Examples: Platyhelminthes, Aschelminthes, and all other higher phyla.
The absence of mesoderm in diploblastic organisms does not mean that the tissues derived from mesoderm are absent. But these are derived either from ectoderm or endoderm. All these two or three germ layers give rise to all kinds of tissues in all organisms.
The germ layers of diploblastic and triploblastic organisms.
The triploblastic organisms are further classified into three basic body construction plans based on Coelom’s presence or absence. The Coelom is a cavity or space between the body wall and internal organs lined with mesoderm. Based on the presence or absence of Coelom, triploblastic organisms are classified into the following types.
- Acoelomate: Organisms without a body cavity other than the gut lumen are called acoelomate. Examples: Platyhelminthes (flatworms).
- Coelomate: Organisms having true mesodermal lined Coelom present are called coelomate. Examples: Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates, and Chordates.
- Pseudocoelomate: The fluid-filled body cavity not lined with mesoderm is called pseudocoel; the organisms having this kind of cavity are called pseudocoelomate. The cavity is derived from blastocoel (an internal space that develops in an embryo before gastrulation). Example: Chelminthes
Schematic representation of Cross section through Acoelomate (left), Coelomate (middle) and Pseudocoelomate (right).
The Coelomates are further classified into two groups based on the method of coelom formation. These are-
- Protostomes: The mouth forms from the first opening that appears during embryonic development. Generally, this opening is a blastopore (opening outside the developing embryo into the archenteron). Examples: Annelida, Mollusca & Arthropoda.
- Deuterostomes: The mouth always develops from an opening other than the blastopore. The blastopore may develop into the anus. Examples: Echinodermata and Chordata.
Functions of Coelom
- The gut is independent of the muscular locomotory functions of the body wall.
- It functions as an internal space to accommodate organs of the digestive system, gonads, embryonic development, etc.
- The internal fluid distributes oxygen, nutrients, hormones, neurosecretory substances, etc.
- It serves as an effective locomotory system.
- Thus, it facilitates the evolution of large body sizes.
Differences between Protostomes and Deuterostomes
- The mouth forms from the first opening that appears during embryonic development. Generally, this opening is the blastopore.
- Coelom formed through schizocoely (gradual enlargement of a small split in mesoderm).
- The number of the coelomic cavity is highly variable. In annelids, the coelomic cavity is equal to the number of body segments.
- The cleavage pattern of eggs is spiral (at an angle of ~45° to the animal-vegetal axis).
- The cleavage is determinate or mosaic (the separated cells cannot develop into whole animals, as the fate of the cells is irrevocably decided after the first cleavage).
- Mesodermal tissue is derived from a single cell (of the 64-cell embryo, located at the edge of the blastopore).
- Sometimes polar lobe is formed in protostomes like a few annelids and molluscs. In gastropod mollusc Crepidula fornicata and plana, the polar lobe formed at the first and second cleavage divisions. [A polar lobe is a small cytoplasm bulge formed before the cell division and does not contain nuclear material. After cell division is complete, the bulge is resorbed into the single daughter cell to which it is still attached]
- The mouth always develops from an opening other than the blastopore, which may develop into the anus.
- Coelom formed through enterocele (evagination of archenteron into embryonic blastocoel; a part of the evagination eventually becomes gut).
- The coelomic cavity is tripartite, i.e., the coelomic cavity is subdivided into three pairs of coelomic pouches.
- The cleavage pattern of eggs is radial (either parallel or perpendicular to the animal-vegetal axis).
- The cleavage is indeterminate or regulative (each separated cell of two or four cell-stage or sometimes eight-cell embryo can develop into a fully functional animal).
- In deuterostomes, the mesoderm is derived from the wall of the archenteron.
- Polar-lobe is never formed in deuterostomes.
Animals are also classified based on the absence or presence of notochord or their derivative. The notochord is a mesodermally derived rod-like structure formed on the dorsal side during the embryonic development of some animals. The animals are classified into two groups based on their absence or presence. These are:
- Non-chordates: Notochord not formed. Example: All phyla from Porifera to Echinodermata.
- Chordates: develop notochord in the embryonic stage and are either retained in some form (in Cephalochordates and Vertebrates) or disappear (in Urochordates).
Some other terms related to animal characterization:
- Metamerism/metameric segmentation: This serial repetition of at least some segments or organ system (skin, musculature, nervous system, circulatory system, reproductive system, and excretory system) is known as metamerism or metameric segmentation. Example: members of phylum Annelida, Arthropoda, and Chordata.
Tagmatization: Highly specialized groups of segments are formed by fusion and modification of different body regions. These segments are called Tagmata (head, thorax, and abdomen). Example: members of phylum Arthropoda.
Moore, J. and Overhill, R., 2006. An Introduction to the Invertebrates. 2nd ed. Leiden: Cambridge University Press.
Pechenik, J., 2015. Biology of the invertebrates. 7th ed. New York, NY: McGraw-Hill.
NCERT. (2019). Biology. National Council of Educational Research and Training.
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