Animal Kingdom

The One Thing

Animal classification compares body organisation, symmetry, germ layers, coelom, segmentation, and notochord to organise animal diversity and identify evolutionary and structural relationships. Major phyla show increasing structural and functional complexity, from cellular organisation in Porifera to organ-system organisation in higher invertebrates and chordates.

Reactions, Processes and Experiments

Key Terms

• Cellular level of organisation: Cells are arranged loosely and perform different functions without forming true tissues; characteristic of Porifera.
• Tissue level of organisation: Similar cells form tissues that perform specific functions; found in Cnidaria and Ctenophora.
• Organ level of organisation: Different tissues combine to form organs; seen in Platyhelminthes.
• Organ-system level of organisation: Organs work together in systems, allowing complex functions; present in higher invertebrates and chordates.
• Body symmetry: The arrangement of body parts around an axis; animals may be asymmetrical, radially symmetrical, or bilaterally symmetrical.
• Asymmetry: The body cannot be divided into two similar halves through any plane; common in most sponges.
• Radial symmetry: Body parts are arranged around a central axis, and the body can be divided into similar halves through several planes; seen in cnidarians and adult echinoderms.
• Bilateral symmetry: The body can be divided into two equal and opposite halves through only one plane; common in most higher animals.
• Diploblastic: The embryo develops two germ layers, ectoderm and endoderm, with a non-cellular mesoglea between them.
• Triploblastic: The embryo develops three germ layers: ectoderm, mesoderm, and endoderm.
• Coelom: A fluid-filled body cavity completely lined by mesoderm.
• Acoelomate: An animal without a body cavity between the body wall and gut; characteristic of Platyhelminthes.
• Pseudocoelomate: An animal with a body cavity that is not completely lined by mesoderm; characteristic of Aschelminthes or Nematoda.
• Coelomate: An animal with a true coelom completely lined by mesoderm; found in Annelida, Mollusca, Arthropoda, Echinodermata, Hemichordata, and Chordata.
• Metamerism: The division of the body into repeated segments; clearly seen in Annelida and also present in Arthropoda and Chordata in modified forms.
• Notochord: A flexible, rod-like supporting structure present in chordate embryos and retained in some adults; it is replaced partly or completely by the vertebral column in vertebrates.
• Porifera: Sponges with cellular organisation, numerous pores, a canal system, and usually an asymmetrical body; examples include Sycon, Spongilla, and Euspongia.
• Cnidaria: Aquatic, diploblastic animals with radial symmetry and stinging cells called cnidoblasts or nematocysts; examples include Hydra, Aurelia, and Adamsia.
• Ctenophora: Marine, diploblastic animals with eight rows of comb plates used for locomotion; examples include Pleurobrachia and Ctenoplana.
• Platyhelminthes: Dorsoventrally flattened, bilaterally symmetrical, triploblastic, acoelomate animals; many are parasitic, such as Taenia and Fasciola.
• Aschelminthes or Nematoda: Unsegmented, bilaterally symmetrical, triploblastic, pseudocoelomate animals with a complete digestive tract; examples include Ascaris, Wuchereria, and Ancylostoma.
• Annelida: Segmented, bilaterally symmetrical, triploblastic, true coelomate animals with organ-system organisation; examples include earthworm, leech, and Nereis.
• Arthropoda: The largest animal phylum, characterised by jointed appendages, a chitinous exoskeleton, segmented body, and usually an open circulatory system; examples include cockroach, honeybee, prawn, and scorpion.
• Mollusca: Soft-bodied, mostly marine animals with a mantle, muscular foot, and visceral mass; many possess a radula; examples include Pila, Unio, Sepia, and Octopus.
• Echinodermata: Exclusively marine animals with a calcareous endoskeleton and water vascular system; adults are radially symmetrical while larvae are bilaterally symmetrical.
• Hemichordata: Marine, worm-like animals with a body divided into proboscis, collar, and trunk; they possess stomochord rather than a true notochord; example: Balanoglossus.
• Chordata: Animals possessing a notochord, dorsal hollow nerve cord, pharyngeal gill slits, and post-anal tail at least during some stage of life.
• Urochordata: Chordates in which the notochord is present mainly in the larval tail; examples include Ascidia and Salpa.
• Cephalochordata: Chordates in which the notochord extends from head to tail and persists throughout life; example: Branchiostoma.
• Vertebrata: Chordates with a vertebral column and cranium; the notochord is generally replaced by the vertebral column in adults.
• Cyclostomata: Jawless, elongated vertebrates with a circular suctorial mouth; examples include Petromyzon and Myxine.
• Chondrichthyes: Cartilaginous fishes with paired fins, a ventral mouth, and no operculum over gills; examples include sharks, skates, and rays.
• Osteichthyes: Bony fishes with an ossified endoskeleton, gill covers, and usually a swim bladder; examples include Rohu, catla, and Hippocampus.
• Amphibia: Vertebrates adapted to life in water and on land, usually with moist skin and aquatic larvae; examples include frog, toad, and salamander.
• Reptilia: Mostly terrestrial vertebrates with dry, keratinised skin and internal fertilisation; examples include lizard, snake, turtle, and crocodile.
• Aves: Warm-blooded, feather-covered vertebrates with wings, beaks, pneumatic bones, and an efficient four-chambered heart; examples include pigeon and sparrow.
• Mammalia: Warm-blooded vertebrates with hair, mammary glands, external ears in most species, and a four-chambered heart; examples include humans, whale, bat, and platypus.

Easily Confused

• Cellular, tissue, organ, and organ-system organisation: These represent progressively higher levels of structural integration, from loosely arranged cells in Porifera to coordinated organs in higher animals.
• Asymmetry, radial symmetry, and bilateral symmetry: Asymmetrical animals cannot be divided into similar halves; radially symmetrical animals can be divided through several planes; bilaterally symmetrical animals can be divided through only one plane.
• Diploblastic and triploblastic: Diploblastic animals have ectoderm and endoderm; triploblastic animals also possess mesoderm.
• Acoelomate, pseudocoelomate, and coelomate: Acoelomates lack a body cavity; pseudocoelomates have a cavity incompletely lined by mesoderm; coelomates have a true cavity completely lined by mesoderm.
• Platyhelminthes and Nematoda: Platyhelminths are acoelomate and commonly have an incomplete digestive tract, whereas nematodes are pseudocoelomate and have a complete digestive tract with a mouth and anus.
• Polyp and medusa: A polyp is usually sessile, whereas a medusa is usually free-swimming.
• Cnidoblasts or nematocysts and colloblasts: Cnidoblasts or nematocysts are stinging cells in cnidarians; colloblasts are adhesive prey-capturing cells in ctenophores.
• Stomochord and notochord: Hemichordates possess a stomochord, whereas chordates possess a true notochord.
• Urochordata and Cephalochordata: In Urochordata the notochord is mainly restricted to the larval tail; in Cephalochordata it extends from head to tail and persists throughout life.
• Chondrichthyes and Osteichthyes: Chondrichthyes have a cartilaginous endoskeleton and no operculum; Osteichthyes have an ossified endoskeleton and gill covers.
• Amphibia and Reptilia: Amphibians generally have moist skin, aquatic larvae, and often require water for reproduction; reptiles have dry keratinised skin and internal fertilisation.
• Bird and mammal adaptations: Birds have feathers, wings, pneumatic bones, and air sacs; mammals have hair and mammary glands.
• Adult and larval echinoderm symmetry: Echinoderm larvae are bilaterally symmetrical, whereas adults are usually radially symmetrical.
• Most reptiles and crocodiles: Most reptiles have a three-chambered heart, but crocodiles are an exception and have a four-chambered heart.

What Gets Asked

• Classification by structural features: Questions may require identification of a phylum from organisation, symmetry, germ layers, coelom, segmentation, or notochord. Marks are lost when a single feature is used without considering the combination of features.
• Comparison of body cavities: Questions may distinguish Platyhelminthes, Aschelminthes or Nematoda, and true coelomates. The key slips are confusing acoelomate with pseudocoelomate or forgetting that the coelom is completely lined by mesoderm.
• Identification of representative organisms: Examples such as Sycon, Hydra, Pleurobrachia, Taenia, Ascaris, earthworm, cockroach, Pila, Balanoglossus, Petromyzon, Rohu, frog, lizard, pigeon, and humans may be matched to their phyla or classes.
• Chordate subdivisions: Questions may compare Urochordata, Cephalochordata, and Vertebrata according to the extent and persistence of the notochord. The common error is treating the notochord as equally persistent in all three groups.
• Vertebrate comparisons: Questions may test gills, lungs, moist or keratinised skin, heart chambers, pneumatic bones, mammary glands, or temperature regulation. Marks are commonly lost by applying the general reptilian three-chambered-heart condition to crocodiles.
• Scientific nomenclature: Questions may ask how representative organisms should be written scientifically. The specific convention is a capitalised genus and lowercase species, as in Homo sapiens.