Animal Diversity: Symmetry, Body Cavities, and Major Taxa

Posted on Dec 17, 2024 in Biology

1. Types of Symmetry in Animals

Radial Symmetry

  • Description: Body parts arranged around a central axis.
  • Examples:
    • Jellyfish (Phylum Cnidaria): Radial symmetry aids in floating and detecting prey from all directions.
    • Sea Anemones (Phylum Cnidaria): Allows equal interaction with the environment on all sides.
    • Starfish (Phylum Echinodermata): Helps in movement and interaction with the environment.
    • Sea Urchins (Phylum Echinodermata): Spines radiate from a central point.
  • Advantages:
    • Can interact with the environment from all directions.
    • Beneficial for sessile or planktonic organisms.
    • Allows efficient capture of food and defense against predators from all angles.
  • Disadvantage:
    • Limited directional movement.
    • Often slower and less efficient in directed movement compared to bilaterally symmetrical animals.

Bilateral Symmetry

  • Description: Body can be divided into equal halves along one plane.
  • Examples:
    • Humans (Phylum Chordata): Aids in coordinated movement and complex behaviors.
    • Dogs (Phylum Chordata): Streamlined body for efficient movement.
    • Fish (Phylum Chordata): Streamlined body for swimming.
    • Butterflies (Phylum Arthropoda): Wing patterns are symmetrical.
  • Advantages:
    • Efficient directional movement.
    • Streamlined body shape.
    • Enhanced sensory integration and coordination.
    • Allows for the development of complex organ systems.
  • Disadvantages:
    • Vulnerable sides.
    • Dependent on a central nervous system for coordination and movement.

Asymmetry

  • Description: No plane of symmetry.
  • Examples:
    • Sponges (Phylum Porifera): Irregular shape, adaptable to various environments.
    • Certain Corals (Phylum Cnidaria): Some have irregular shapes.
    • Amorphous Protozoans (Single-celled Organisms): Lack a defined shape.
  • Advantages:
    • Flexible body structure.
    • Can adapt to different and irregular environments.
    • Less energy required for maintaining symmetry.
  • Disadvantages:
    • Lack of structured movement.
    • Limited ability to coordinate complex movements.
    • Often leads to simple body plans.

2. Body Cavities: Acoelomate, Pseudocoelomate, and Coelomate

Acoelomate

  • Description: No body cavity; organs embedded in solid tissue.
  • Examples:
    • Flatworms (Phylum Platyhelminthes): Simple body plan, solid body.
    • Planarians: Acoelomate structure, free-living.
    • Tapeworms: Parasites with acoelomate bodies, lack digestive systems.
    • Flukes: Parasitic, complex life cycles.
  • Characteristics: Simplified body plan, direct contact between tissues and environment.
  • Advantages:
    • Simpler structure, less energy required for development.
    • Close proximity of organs facilitates diffusion.
    • Flexibility in body shape and movement.
  • Disadvantages:
    • Limited organ complexity and specialization.
    • Less efficient in nutrient and waste transport.
    • Lack of a true circulatory system.

Pseudocoelomate

  • Description: False body cavity; cavity not fully lined with mesoderm.
  • Examples:
    • Roundworms (Phylum Nematoda): Fluid-filled pseudocoelom, diverse habitats.
    • Rotifers (Phylum Rotifera): Microscopic pseudocoelomates, wheel-like cilia.
    • Heartworms: Parasites in animals, live in blood vessels.
    • Hookworms: Parasitic, feed on host blood.
  • Characteristics: Cavity between endoderm and mesoderm, acts as a hydrostatic skeleton.
  • Advantages:
    • Improved nutrient and waste transport.
    • Provides space for the development of more complex organs.
    • Acts as a hydrostatic skeleton for movement.
  • Disadvantages:
    • Less structural support compared to a true coelom.
    • Limited capacity for organ specialization and development.
    • Dependence on a fluid-filled cavity for support.

Coelomate

  • Description: True body cavity; cavity fully lined with mesoderm.
  • Examples:
    • Earthworms (Phylum Annelida): Coelom allows for complex organs, segmented bodies.
    • Humans (Phylum Chordata): Coelom supports complex internal organs, including the heart and lungs.
    • Mollusks (Phylum Mollusca): Coelom for organ development, includes snails, clams, and octopuses.
    • Arthropods (Phylum Arthropoda): Coelomate with a highly developed body plan.
  • Characteristics: Fully lined body cavity, enables the development of complex organ systems.
  • Advantages:
    • Enhanced organ function and specialization.
    • Better support for internal structures and organs.
    • Allows for the development of more complex body plans and behaviors.
    • Efficient nutrient and waste transport through a closed circulatory system.
  • Disadvantages:
    • More energy required for development and maintenance.
    • Complexity can lead to more points of failure in the organism’s physiology.

3. Characteristics of Major Taxa

Phylum Porifera (Sponges)

  • Characteristics: No true tissues, porous body, filter feeders, sessile lifestyle, composed of spicules and spongin, capable of asexual (budding) and sexual reproduction.
  • Examples:
    • Bath Sponges: Commonly used in homes, flexible structure.
    • Glass Sponges: Found in deep sea, silica-based spicules.
    • Calcareous Sponges: Made of calcium carbonate, found in shallow marine environments.

Phylum Cnidaria

  • Characteristics: Radial symmetry, stinging cells (cnidocytes), two body forms (polyp and medusa), gastrovascular cavity, nerve net, hydrostatic skeleton.
  • Examples:
    • Jellyfish: Medusa form, swimming, gelatinous body.
    • Sea Anemones: Polyp form, attached to surfaces, colorful tentacles.
    • Corals: Colonial polyps, build reefs, symbiotic relationship with zooxanthellae.

Class Hydrozoa

  • Characteristics: Alternates between polyp and medusa forms, often colonial, simple structure, presence of nematocysts.
  • Examples:
    • Hydra: Solitary, freshwater, can regenerate.
    • Portuguese Man O’ War: Colonial, marine, floating siphonophore.
    • Obelia: Colonial, marine, both polyp and medusa stages.

Class Anthozoa (Sea Anemones, Corals)

  • Characteristics: Only polyp form, often colonial, calcium carbonate skeleton (in corals), diverse shapes and sizes, symbiosis with algae.
  • Examples:
    • Sea Anemones: Solitary polyps, colorful, symbiotic relationships with clownfish.
    • Stony Corals: Build coral reefs, hard exoskeleton, vital for marine biodiversity.
    • Soft Corals: Flexible, not reef-building, often fan-shaped.

Class Scyphozoa (Jellies)

  • Characteristics: Dominant medusa stage, thick mesoglea, complex life cycle, some bioluminescent.
  • Examples:
    • Moon Jellyfish: Common jellyfish, transparent, mild sting.
    • Lion’s Mane Jellyfish: Large, long tentacles, cold water habitats.
    • Box Jellyfish: Venomous, cube-shaped medusa, potent sting.

Phylum Ctenophora (Comb Jellies)

  • Characteristics: Eight rows of ciliary plates (combs) for locomotion, bioluminescence, sticky tentacles (colloblasts), lack cnidocytes.
  • Examples:
    • Sea Gooseberry: Transparent, marine, spherical body.
    • Venus Girdle: Long, ribbon-like, bioluminescent.
    • Pleurobrachia: Round body, two long tentacles, common in coastal waters.

Protostomes

  • Characteristics: Mouth develops before the anus, spiral and determinate cleavage, coelom formation by splitting (schizocoely).
  • Examples:
    • Annelids: Segmented worms, coelomate, closed circulatory system.
    • Mollusks: Soft-bodied, often with hard shells, diverse body forms.
    • Arthropods: Exoskeleton, segmented bodies, jointed appendages, largest phylum.

Phylum Platyhelminthes (Flatworms)

  • Characteristics: Bilateral symmetry, acoelomate, dorsoventrally flattened body, simple digestive system, hermaphroditic.
  • Examples:
    • Planarians: Free-living, freshwater, regeneration abilities.
    • Tapeworms: Parasitic, segmented body, live in intestines of vertebrates.
    • Flukes: Parasitic, complex life cycles, infect various organs.

Phylum Mollusca

  • Characteristics: Soft bodies, often with a hard shell, mantle, muscular foot, radula (in most), coelomate.
  • Examples:
    • Snails: Gastropods with coiled shells, diverse habitats.
    • Clams: Bivalves with two-part shells, filter feeders.
    • Octopuses: Cephalopods, highly intelligent, eight arms.

Class Gastropoda (Snails, Slugs)

  • Characteristics: Torsion during development, muscular foot, diverse feeding habits, often have coiled shells.
  • Examples:
    • Garden Snails: Common land snails, spiral shells.
    • Sea Slugs: Marine, colorful, often lack shells.
    • Conchs: Large shells, marine, edible.

Class Bivalvia (Clams, Oysters)

  • Characteristics: Two-part shell, filter feeders, reduced head, sedentary lifestyle, siphons for water flow.
  • Examples:
    • Mussels: Attached to surfaces, strong byssal threads.
    • Scallops: Mobile, swim by clapping shells, edible.
    • Oysters: Produce pearls, reef builders, important for water filtration.

Class Cephalopoda (Octopuses, Squids)

  • Characteristics: Highly developed nervous system, tentacles with suckers, beak-like jaws, excellent vision, capable of complex behaviors.
  • Examples:
    • Cuttlefish: Camouflage abilities, internal shell (cuttlebone).
    • Nautilus: Chambered shell, ancient lineage.
    • Squids: Fast swimmers, elongated bodies, ink sacs for defense.

Phylum Annelida (Segmented Worms)

  • Characteristics: Segmented bodies, coelomate, setae or bristles, closed circulatory system, diverse habitats.
  • Examples:
    • Earthworms: Soil-dwelling, important for soil aeration and nutrient cycling.
    • Leeches: Parasitic, produce anticoagulants, used in medical treatments.
    • Polychaetes: Marine worms, parapodia for movement, diverse forms.

Phylum Lophophorata

  • Characteristics: Lophophore (ciliated feeding structure), coelomate, often sessile, filter feeders.
  • Examples:
    • Brachiopods: Shells similar to bivalves, marine, ancient lineage.
    • Bryozoans: Colonial, encrusting, form intricate colonies.
    • Phoronids: Worm-like, tube-dwelling, marine.

Phylum Rotifera

  • Characteristics: Microscopic, pseudocoelomate, wheel-like cilia (corona) for feeding and locomotion, diverse reproductive strategies.
  • Examples:
    • Rotifers: Found in freshwater and marine environments, diverse shapes.
    • Bdelloids: Asexual reproduction, known for desiccation resistance.
    • Monogononts: Complex life cycles, sexual and asexual reproduction.

Phylum Nematoda (Roundworms)

  • Characteristics: Cylindrical bodies, pseudocoelomate, complete digestive system, covered by a cuticle, diverse habitats.
  • Examples:
    • Caenorhabditis elegans: Model organism in genetic studies.
    • Parasitic Nematodes: Affect plants and animals, significant agricultural pests.
    • Heartworms: Infect dogs and other animals, transmitted by mosquitoes.

Phylum Tardigrada (Water Bears)

  • Characteristics: Microscopic, segmented bodies, able to survive extreme conditions (cryptobiosis), four pairs of legs with claws.
  • Examples:
    • Tardigrades: Found in diverse environments, from deep sea to mountaintops.
    • Hypsibius: Common species, studied for cryptobiosis.
    • Echiniscus: Tardigrade genus with distinctive armor plates.

Phylum Onychophora (Velvet Worms)

  • Characteristics: Segmented bodies, slime glands for capturing prey, soft-bodied, terrestrial, closely related to arthropods.
  • Examples:
    • Peripatus: Common velvet worm, found in tropical forests.
    • Epiperipatus: Genus of velvet worms, varied habitats.
    • Macroperipatus: Genus of large velvet worms.

Phylum Arthropoda

  • Characteristics: Exoskeleton made of chitin, segmented bodies, jointed appendages, most diverse phylum, open circulatory system.
  • Examples:
    • Insects: Largest class, six legs, wings, diverse forms and habitats.
    • Spiders: Arachnids, eight legs, silk production.
    • Crabs: Crustaceans, two pairs of antennae, marine and terrestrial species.

Subphylum Myriapoda

  • Characteristics: Many legs, elongated bodies, two main groups (centipedes and millipedes).
  • Examples:
    • Centipedes: Carnivorous, one pair of legs per segment, venomous claws.
    • Millipedes: Detritivores, two pairs of legs per segment, cylindrical bodies.
    • Symphylans: Soil-dwelling, small, resemble centipedes.

Subphylum Chelicerata

  • Characteristics: Chelicerae (mouthparts), two body segments (cephalothorax and abdomen).
  • Examples:
    • Spiders: Venomous fangs, silk production.
    • Scorpions: Venomous sting, pincers.
    • Horseshoe Crabs: Marine, ancient lineage.

Subphylum Crustacea

  • Characteristics: Mostly aquatic, two pairs of antennae, biramous (branched) appendages.
  • Examples:
    • Crabs: Strong pincers, various habitats.
    • Lobsters: Large claws, deep sea.
    • Shrimp: Swimmerets for movement, commercial importance.

Subphylum Insecta

  • Characteristics: Three body segments (head, thorax, abdomen), three pairs of legs, usually two pairs of wings.
  • Examples:
    • Beetles: Hard exoskeleton, diverse species.
    • Butterflies: Colorful wings, metamorphosis.
    • Ants: Social structure, strong mandibles.

Deuterostomata

  • Characteristics: Anus develops before the mouth, radial cleavage.
  • Examples:
    • Echinoderms: Radial symmetry in adults.
    • Chordates: Notochord, dorsal nerve cord.
    • Hemichordates: Worm-like, marine.

Phylum Echinodermata

  • Characteristics: Radial symmetry in adults, water vascular system.
  • Examples:
    • Sea Stars: Regeneration abilities, tube feet.
    • Sea Urchins: Spiny exterior, grazing habits.
    • Sand Dollars: Flattened, burrowing lifestyle.

Phylum Chordata

  • Characteristics: Notochord, dorsal nerve cord, pharyngeal slits, post-anal tail.
  • Examples:
    • Tunicates: Marine, filter feeders.
    • Lancelets: Fish-like, burrow in sand.
    • Vertebrates: Backbone, complex systems.

Subphylum Urochordata (Tunicates)

  • Characteristics: Marine, filter feeders, notochord in larval stage.
  • Examples:
    • Sea Squirts: Sessile adults, siphons.
    • Salps: Free-floating, gelatinous.
    • Larvaceans: Resemble tadpoles, mucus houses.

Subphylum Cephalochordata (Lancelets)

  • Characteristics: Fish-like, retain notochord throughout life.
  • Examples:
    • Branchiostoma: Burrowing lifestyle.
    • Asymmetron: Found in shallow waters.
    • Epigonichthys: Marine habitats.

Subphylum Vertebrata

  • Characteristics: Backbone, complex organ systems.
  • Examples:
    • Jawless Fish (Agnatha): Lampreys, hagfish.
    • Jawed Fish: Sharks, bony fish.
    • Tetrapods: Amphibians, reptiles, mammals.

Jawless Fish (Agnatha)

  • Characteristics: No jaws, cartilaginous skeleton.
  • Examples:
    • Lampreys: Parasitic, eel-like.
    • Hagfish: Scavengers, slime production.
    • Myxini: Hagfish group.

Jawed Fish

  • Characteristics: Jaws, paired fins, scales.
  • Examples:
    • Sharks: Cartilaginous skeletons, predatory.
    • Rays: Flattened bodies, pectoral fins.
    • Bony Fish: Ossified skeletons, swim bladders.

Class Amphibia

  • Characteristics: Dual life (aquatic larval stage, terrestrial adult stage), moist skin for respiration.
  • Examples:
    • Frogs: Jumping, vocalization.
    • Salamanders: Regeneration, tails.
    • Caecilians: Limbless, burrowing.

Class Reptilia

  • Characteristics: Scaly skin, lay eggs on land, ectothermic.
  • Examples:
    • Lizards: Diverse habitats, varied diets.
    • Snakes: Limbless, some venomous.
    • Turtles: Shells, aquatic and terrestrial.

Class Mammalia

  • Characteristics: Hair or fur, mammary glands, endothermic.
  • Examples:
    • Primates: Opposable thumbs, complex social structures.
    • Rodents: Gnawing teeth, diverse habitats.
    • Cetaceans: Aquatic, echolocation.