Diversity in Animal Kingdom

The One Thing

Animal classification groups organisms according to shared structural, developmental, and functional features. The principal comparisons concern body organisation, symmetry, germ layers, body cavity, segmentation, and the presence or absence of a notochord, producing a broad division between non-chordates and chordates.

Reactions, Processes and Experiments

Key Terms

• Classification: The systematic arrangement of organisms into groups based on similarities and differences.
• Animal kingdom: The group of multicellular, generally motile organisms that obtain food from other organisms.
• Body organisation: The level at which the body is organised, such as cellular, tissue, organ, or organ-system level.
• Cellular level: A simple organisation in which cells perform different functions but are not arranged into true tissues, as in sponges.
• Tissue level: An organisation in which similar cells form tissues that carry out specific functions, as in cnidarians.
• Organ level: An organisation in which tissues combine to form organs, seen in some flatworms.
• Organ-system level: The highest level in which organs work together as systems, found in animals such as annelids, arthropods, molluscs, echinoderms, and chordates.
• Symmetry: The arrangement of body parts around a central point or axis.
• Asymmetry: A condition in which the body cannot be divided into similar halves through any plane, as in most sponges.
• Radial symmetry: A condition in which several planes passing through the central axis divide the body into similar parts, as in cnidarians and adult echinoderms.
• Bilateral symmetry: A condition in which only one plane divides the body into two equal halves, as in flatworms and humans.
• Germ layers: Embryonic layers that develop into different tissues and organs.
• Diploblastic: Having two germ layers, ectoderm and endoderm, with a non-cellular mesoglea between them.
• Triploblastic: Having three germ layers: ectoderm, mesoderm, and endoderm.
• Body cavity or coelom: A fluid-filled space between the body wall and the digestive tract that provides room for internal organs.
• Acoelomate: An animal without a true body cavity, as in flatworms.
• Pseudocoelomate: An animal with a body cavity that is not completely lined by mesoderm, as in roundworms.
• Coelomate: An animal with a true body cavity completely lined by mesoderm, as in annelids and chordates.
• Segmentation: The division of the body into repeated segments, seen clearly in annelids, arthropods, and chordates.
• Notochord: A flexible supporting rod present at least during some stage of development in chordates.
• Invertebrates: Animals that do not possess a vertebral column, including sponges, cnidarians, worms, molluscs, arthropods, and echinoderms.
• Vertebrates: Chordates with a vertebral column and a well-developed internal skeleton.
• Porifera: Sponges with numerous pores, a canal system, cellular organisation, and a body generally fixed to a surface.
• Cnidaria or Coelenterata: Mostly aquatic animals with radial symmetry, tissue-level organisation, and stinging cells called cnidoblasts.
• Platyhelminthes: Flatworms with bilateral symmetry, a flattened body, triploblastic organisation, and no true body cavity.
• Aschelminthes or Nematoda: Roundworms with a cylindrical, unsegmented body and a pseudocoelom; many are parasitic.
• Annelida: Segmented worms with bilateral symmetry, a true coelom, and organ-system organisation.
• Arthropoda: The largest animal phylum, having jointed appendages, a segmented body, and a chitinous exoskeleton.
• Mollusca: Soft-bodied animals usually having a muscular foot, visceral mass, mantle, and often a shell.
• Echinodermata: Marine animals with spiny skin, a water vascular system, and radial symmetry in adults.
• Hemichordata: Marine animals with a body divided into proboscis, collar, and trunk; they possess a stomochord but not a true notochord.
• Chordata: Animals possessing a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at least during some stage.
• Urochordata: Chordates in which the notochord is present mainly in the larval tail, as in tunicates.
• Cephalochordata: Chordates in which the notochord extends from the head to the tail and persists throughout life.
• Vertebrata: Chordates in which the notochord is replaced partly or completely by a vertebral column.
• Pisces: Aquatic vertebrates with gills, fins, scales in most species, and a generally two-chambered heart.
• Amphibia: Vertebrates that commonly live both in water and on land; they usually have moist skin and undergo metamorphosis.
• Reptilia: Mostly land-dwelling vertebrates with dry, scaly skin and lungs for respiration.
• Aves: Warm-blooded vertebrates with feathers, wings, beaks, and lightweight bones adapted for flight in most species.
• Mammalia: Warm-blooded vertebrates with hair or fur and mammary glands that produce milk for young ones.
• Cold-blooded or ectothermic: Animals that depend largely on environmental temperature to regulate body temperature.
• Warm-blooded or endothermic: Animals that maintain a relatively constant internal temperature.

Easily Confused

• Asymmetry vs radial symmetry: Asymmetrical bodies cannot be divided into similar halves through any plane, whereas radial symmetry permits several planes through the central axis to produce similar parts.
• Radial symmetry vs bilateral symmetry: Radial symmetry permits several planes of division, whereas bilateral symmetry permits only one plane dividing the body into equal halves.
• Diploblastic vs triploblastic: Diploblastic animals have ectoderm and endoderm, whereas triploblastic animals also possess mesoderm.
• Acoelomate vs pseudocoelomate: Acoelomates lack a true body cavity, whereas pseudocoelomates possess a cavity that is not completely lined by mesoderm.
• Pseudocoelomate vs coelomate: A pseudocoelomate has an incompletely mesoderm-lined cavity, whereas a coelomate has a true cavity completely lined by mesoderm.
• Stomochord vs notochord: Hemichordates possess a stomochord but not a true notochord; chordates possess a notochord at least during some stage.
• Urochordata vs Cephalochordata: In Urochordata the notochord is mainly in the larval tail, whereas in Cephalochordata it extends from head to tail and persists throughout life.
• Invertebrates vs vertebrates: Invertebrates lack a vertebral column, whereas vertebrates possess a vertebral column and a well-developed internal skeleton.
• Fish vs amphibians: Fish generally use gills and have a two-chambered heart, whereas adult amphibians use lungs and skin and generally have a three-chambered heart.
• Amphibians vs reptiles: Amphibians generally have moist skin, undergo metamorphosis, and often lay eggs in water; reptiles have dry, scaly skin and are mainly land-dwelling.
• Reptiles vs birds: Reptiles generally have dry scales and a three-chambered heart, whereas birds have feathers and a four-chambered heart.
• Cold-blooded vs warm-blooded: Cold-blooded animals depend largely on environmental temperature, whereas warm-blooded animals maintain a relatively constant internal temperature.

What Gets Asked

• Questions may require the sequence of increasing body organisation: cellular level, tissue level, organ level, and organ-system level. A common error is to omit or reverse one of these levels.
• Classification questions may ask for the sequence from simpler non-chordates to chordates: Porifera, Cnidaria, Platyhelminthes, Aschelminthes or Nematoda, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata. The named phyla must be kept in the correct order.
• Comparative questions may ask students to distinguish acoelomate, pseudocoelomate, and coelomate conditions. The mark-losing error is confusing an incompletely lined cavity with a cavity completely lined by mesoderm.
• Questions may require the four defining chordate features: notochord, dorsal hollow nerve cord, pharyngeal gill slits, and post-anal tail. These should not be replaced by the vertebral column, which characterises vertebrates.
• Identification questions may use examples such as Sycon and Spongilla, Hydra, jellyfish, sea anemone, and coral, Taenia and Planaria, Ascaris and Wuchereria, earthworm and leech, cockroach, prawn, spider, and butterfly, snail, oyster, and octopus, and starfish and sea urchin. The error is assigning an example to the wrong phylum.
• Vertebrate comparison questions may test defining features and exceptions, especially the four-chambered heart of crocodiles despite the generally three-chambered heart of reptiles, and the distinction between the two-chambered heart of most fish, the three-chambered heart of amphibians, and the four-chambered hearts of birds and mammals.