Introduction
The animal kingdom is a fascinating and diverse domain, encompassing a myriad of species with distinct structures and forms. With over a million described species, the classification of animals becomes essential for understanding and organizing this diversity. Classification not only helps in systematic organization but also aids in identifying and categorizing new species.
Basis of Classification
Despite the wide variety of structures and forms, animals share certain fundamental features. These features, which form the basis of classification, include the arrangement of cells, body symmetry, nature of the coelom, and patterns of the digestive, circulatory, and reproductive systems.
Levels of Organisation
All animals are multicellular, but they exhibit different levels of cellular organization:
- Cellular Level: In sponges, cells are loosely aggregated without true tissues.
- Tissue Level: In coelenterates, similar cells are organized into tissues.
- Organ Level: In Platyhelminthes and higher phyla, tissues form organs specialized for specific functions.
- Organ-System Level: In Annelids, Arthropods, Molluscs, Echinoderms, and Chordates, organs form functional systems for specific physiological processes.
Symmetry
Animals can be categorized based on symmetry:
- Asymmetrical: No symmetry, as seen in sponges.
- Radial Symmetry: Symmetry around a central axis, typical of coelenterates, ctenophores, and adult echinoderms.
- Bilateral Symmetry: Body can be divided into identical left and right halves, as in annelids, arthropods, and vertebrates.
Diploblastic and Triploblastic Organisation
- Diploblastic: Animals with two germ layers—ectoderm and endoderm (e.g., coelenterates).
- Triploblastic: Animals with three germ layers—ectoderm, mesoderm, and endoderm (e.g., platyhelminthes to chordates).
Coelom
The presence or absence of a body cavity (coelom) is crucial in classification:
- Coelomates: Animals with a true coelom lined by mesoderm (e.g., annelids, molluscs, arthropods).
- Pseudocoelomates: Animals with a body cavity not fully lined by mesoderm (e.g., aschelminthes).
- Acoelomates: Animals without a body cavity (e.g., platyhelminthes).
Segmentation
Some animals exhibit segmentation, where the body is divided into repeated segments both internally and externally (e.g., earthworms).
Notochord
The presence of a notochord, a mesodermally derived rod-like structure, is a key feature:
- Chordates: Animals with a notochord (e.g., vertebrates).
- Non-chordates: Animals without a notochord (e.g., invertebrates).
Classification of Animals
The animal kingdom can be broadly classified based on the features discussed above.
Phylum Porifera
- Commonly Known As: Sponges
- Characteristics: Marine, asymmetrical, cellular level of organization, water transport system, intracellular digestion, skeleton of spicules or spongin fibers.
- Examples: Sycon, Spongilla, Euspongia
Phylum Coelenterata (Cnidaria)
- Characteristics: Aquatic, radial symmetry, tissue level of organization, cnidocytes for defense and prey capture, gastrovascular cavity, extracellular and intracellular digestion.
- Examples: Physalia, Adamsia, Pennatula, Gorgonia, Meandrina
Phylum Ctenophora
- Commonly Known As: Sea walnuts or comb jellies
- Characteristics: Marine, radial symmetry, tissue level of organization, ciliated comb plates for locomotion, bioluminescence.
- Examples: Pleurobrachia, Ctenoplana
Phylum Platyhelminthes
- Commonly Known As: Flatworms
- Characteristics: Bilateral symmetry, triploblastic, acoelomate, organ level of organization, parasitic forms with hooks and suckers, high regeneration capacity.
- Examples: Taenia, Fasciola
Phylum Aschelminthes
- Commonly Known As: Roundworms
- Characteristics: Bilateral symmetry, triploblastic, pseudocoelomate, complete alimentary canal, separate sexes.
- Examples: Ascaris, Wuchereria, Ancylostoma
Phylum Annelida
- Characteristics: Bilateral symmetry, triploblastic, coelomate, segmented body, closed circulatory system, nephridia for excretion.
- Examples: Nereis, Pheretima, Hirudinaria
Phylum Arthropoda
- Characteristics: Largest phylum, bilateral symmetry, triploblastic, segmented, chitinous exoskeleton, jointed appendages, open circulatory system.
- Examples: Apis, Bombyx, Laccifer, Anopheles, Locusta, Limulus
Phylum Mollusca
- Characteristics: Bilateral symmetry, triploblastic, coelomate, calcareous shell, unsegmented body, mantle cavity with gills, radula for feeding.
- Examples: Pila, Pinctada, Sepia, Octopus, Aplysia, Dentalium, Chaetopleura
Phylum Echinodermata
- Characteristics: Marine, organ-system level of organization, radial symmetry in adults, bilateral symmetry in larvae, water vascular system.
- Examples: Asterias, Echinus, Antedon, Cucumaria, Ophiura
Phylum Hemichordata
- Characteristics: Previously considered a sub-phylum, bilaterally symmetrical, triploblastic, coelomate, body divided into proboscis, collar, and trunk, open circulatory system, complete digestive system.
- Examples: Balanoglossus, Saccoglossus
Phylum – Chordata: Overview
Animals in the phylum Chordata are defined by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits. They are bilaterally symmetrical, triploblastic, and coelomate with an organ-system level of organization. Key features include a post-anal tail and a closed circulatory system.
Chordates are divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata. Urochordates (e.g., Ascidia) have a notochord only in the larval tail, while cephalochordates (e.g., Branchiostoma) retain it throughout life. Vertebrates possess a notochord during the embryonic stage, replaced by a vertebral column in adults. They have a ventral heart, kidneys, and paired appendages.
Chordates exhibit significant diversity and include various classes such as Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia, each with distinct characteristics and adaptations for survival in different habitats.
This comprehensive overview of the animal kingdom highlights the incredible diversity and complexity of animal life, providing a systematic framework for understanding the relationships and characteristics of different animal groups.
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