Comprehensive Biology Notes on Animal Classification and Symmetry

Patterns of Animal Organization and Symmetry

  • Symmetry is defined as the arrangement of body parts of an animal around a point or a central axis.

  • The concept of symmetry is fundamental for understanding the organization of an animal's body.

  • There are three primary types of animal symmetry:

    • Asymmetry

    • Radial symmetry

    • Bilateral symmetry

  • Asymmetry:

    • Definition: The arrangement of body parts without a central axis or point.

    • Characteristics: This refers to a lack of symmetry, meaning the body does not have a consistent arrangement of parts.

    • Example: Sponges.

  • Radial Symmetry:

    • Definition: The arrangement of body parts such that any plane passing through the oral-aboral axis divides the animal into mirror images.

    • Characteristics: Body parts radiate outwards from a central point, allowing for multiple planes of symmetry.

    • Modified Radial Symmetries:

      • Biradial Symmetry: The arrangement of some structures in pairs or other combinations around the central axis (e.g., ctenophorans and some anthozoans).

      • Pentaradial Symmetry: The arrangement of structures in combinations of five around a central axis (e.g., echinoderms).

    • Example: Cnidarians.

  • Bilateral Symmetry:

    • Definition: The arrangement of body parts such that a single plane passing between the upper and lower surfaces and through the longitudinal axis divides the animal into right and left mirror images.

    • Characteristics: Characterized by a single plane of symmetry, dividing the body into two mirror-image halves.

    • Example: Vertebrates.

Detailed Anatomical Terminology and Descriptions

  • Aboral: The end opposite the mouth of a radially symmetrical animal.

  • Oral: The end containing the mouth of a radially symmetrical animal.

  • Anterior: The head end; usually the end of a bilateral animal that meets its environment.

  • Posterior: The tail end.

  • Caudal: Toward the tail.

  • Cephalic: Toward the head.

  • Distal: Away from the point of attachment of a structure on the body (e.g., the toes are distal to the knee).

  • Proximal: Toward the point of attachment of a structure on the body (e.g., the hip is proximal to the knee).

  • Dorsal: The back of an animal, usually the upper surface. In animals that walk upright, this is synonymous with posterior.

  • Ventral: The belly of an animal, usually the lower surface. In animals that walk upright, this is synonymous with anterior.

  • Inferior: Below a point of reference (e.g., the mouth is inferior to the nose in humans).

  • Superior: Above a point of reference (e.g., the neck is superior to the chest).

  • Lateral: Away from the plane that divides a bilateral animal into mirror images.

  • Medial (Median): On or near the plane that divides a bilateral animal into mirror images.

Primary Criteria for Animal Classification

  • Animals are classified according to three main categories: their structure, the food they eat, and their mode of reproduction.

  • Classification According to Structure:

    • Invertebrates:

      • They do not have a backbone.

      • They represent over 90%90\% of all animals on Earth.

      • Main groups: Arthropods, mollusks, worms, echinoderms, sponges.

    • Vertebrates:

      • They possess a backbone.

      • They represent less than 10%10\% of all animals on Earth.

      • Main groups: Mammals, birds, fish, reptiles, amphibians.

  • Classification According to Diet:

    • Carnivorous: Animals that eat other animals.

    • Herbivorous: Animals that eat plants and fruits.

    • Omnivorous: Animals that eat both plants and animals.

  • Classification According to Reproduction and Birth:

    • Viviparous: They grow inside their mother until they are born from the mother's stomach.

    • Oviparous: They are born from eggs. They lay eggs, and babies grow inside the eggs until they are ready to hatch.

    • Ovoviviparous: They develop within eggs that remain protected inside the mother’s body until they are ready to be born.

Systematic Survey of Invertebrates

  • Protozoa:

    • Single-cell organisms.

    • All are microscopic.

  • Flatworms:

    • Simple and soft-bodied.

    • Examples: Tape worm, Flukes.

  • Annelid Worms:

    • Characterized by segmented bodies.

    • Examples: Earthworm, Leech.

  • Echinoderms:

    • Spiny sea creatures.

    • Examples: Starfish, Sea urchin.

  • Coelenterates:

    • Soft bodies with stinging cells.

    • Examples: Jellyfish, Sea anemone.

  • Molluscs:

    • Soft-bodied animals; most have shells.

    • Examples: Snails, Limpet.

  • Arthropods:

    • Feature a hard external skeleton (exoskeleton) and jointed limbs.

    • Sub-groups of Arthropods:

      • Arachnids: Eight legs, two body parts, and no antennae (e.g., Spider, Scorpion).

      • Crustaceans: Mostly sea creatures with many legs and two sets of antennae (e.g., Crab, Lobster).

      • Insects: Wings, six legs, three body parts, and one pair of antennae (e.g., Bee, Ladybird).

      • Myriapods: Many legs and multiple body segments (e.g., Centipede, Millipede).

Systematic Survey of Vertebrates

  • Blood Temperature Characteristics:

    • Cold-blooded (Ectothermic): Animals that depend on their surroundings to maintain body temperature. They get cold when there is no sun and active when it is hot.

    • Warm-blooded (Endothermic): Animals that regulate their own body temperature. They normally have layers of fat, feathers, or fur to assist in insulation.

  • Amphibians:

    • Blood-type: Cold-blooded.

    • Habitat: Live both on land and in water (live in water as tadpoles and adults on land).

    • Respiration: Breathe through gills as tadpoles and with lungs as adults; also have moist smooth skin.

    • Physical Features: Moist slimy skin, webbed feet, and typically 4 legs (though sometimes none).

    • Reproduction: Lay many jelly-coated eggs in water.

    • Examples: Frogs, toads, salamanders, newts.

  • Birds:

    • Blood-type: Warm-blooded.

    • Physical Features: Feathers, wings, hollow bones, two legs, and ear holes instead of external ears.

    • Respiration: Breathe using lungs.

    • Reproduction: Lay hard-shelled eggs. Because birds are warm-blooded, eggs must be incubated to keep embryos warm or they will perish.

    • Flight: Most can fly, though some like penguins and ostriches cannot.

    • Examples: Wren, swan, ostrich, penguin.

  • Fish:

    • Blood-type: Cold-blooded (with some exceptions).

    • Habitat: Live in water.

    • Physical Features: Scales, fins, and wet scales (no hair or fur).

    • Respiration: Breathe underwater using gills, not lungs.

    • Reproduction: Most lay many eggs in water (though sharks give birth to live young).

    • Examples: Shark, tuna.

  • Reptiles:

    • Blood-type: Cold-blooded.

    • Habitat: Found more commonly in tropics; not found in polar areas.

    • Physical Features: Dry scaly skin, four legs (usually), or no legs (snakes), and ear holes instead of external ears.

    • Respiration: Breathe with lungs.

    • Reproduction: Usually lay leathery-shelled eggs on dry land, though some give birth to live young.

    • Historic Example: Dinosaurs were reptiles, though some scientists believe a few were warm-blooded.

    • Modern Examples: Snake, crocodile, turtle, tortoise, alligator, lizard.

  • Mammals:

    • Blood-type: Warm-blooded.

    • Physical Features: Have hair or fur, ears that stick out (for land-dwellers), and four legs (land-dwellers).

    • Respiration: Have lungs and need air to breathe.

    • Reproduction and Care: Mothers nurse their young with milk; most give birth to live offspring (viviparous).

    • Exception: The platypus is a mammal that lays eggs.

    • Examples: Human, cow, platypus.