Simple Animals & Fundamental Animal Biology

Fundamental Characteristics of Animals

  • Eukaryotic cells with membrane-bound nucleus and organelles.
  • Lack rigid cell walls ➔ allows flexibility & movement.
  • Multicellular organization; huge diversity of specialized cell types.
  • Heterotrophic lifestyle ➔ must obtain organic food from other organisms.
    • Cell specialization: each cell’s shape, organelles & biochemistry suit a specific task.
    • Division of labour: essential survival tasks divided among specialized cells/tissues.

Seven Universal Tasks for Animal Survival

  • Feeding
    • Herbivores: consume roots, stems, leaves, flowers, seeds, fruit.
    • Carnivores: consume other animals (fat, muscle, bone marrow, blood).
    • Parasites: live on/in another organism; may harm or (rarely) benefit host.
    • Filter feeders: aquatic; strain microscopic food from water (e.g. sponges, some worms).
    • Detritus feeders: ingest decaying organic matter; act as decomposers.
  • Respiration
    • Cellular equation: C<em>6H</em>12O<em>6+6O</em>2→6CO<em>2+6H</em>2O+36ATP\mathrm{C<em>6H</em>{12}O<em>6 + 6O</em>2 \rightarrow 6CO<em>2 + 6H</em>2O + 36ATP}.
    • Small/flat animals exchange gases across body surface.
    • Large animals need lungs or gills + internal transport.
    • Respiratory pigments: hemoglobin (Fe-based, red); hemocyanin (Cu-based, blue).
  • Internal Transport
    • Complex animals use circulatory systems to move $O2$, $CO2$, nutrients & wastes via blood vessels.
  • Excretion
    • Simple aquatic animals: diffusion straight to environment.
    • Larger forms: specialized excretory organs to remove N-wastes & ions.
  • Response
    • Specialized sensory cells (eyes, ears, mechanoreceptors, chemoreceptors).
    • Detect predators, food, mates; nervous system processes stimuli.
  • Movement
    • Sessile: fixed to substrate for life (e.g. adult sponges, coral polyps).
    • Motile: move via muscular–skeletal systems.
    • Exoskeleton (arthropods); Endoskeleton (reptiles, birds, mammals).
  • Reproduction & Development
    • Asexual: budding, fragmentation, parthenogenesis.
    • Sexual: internal vs external fertilization.
    • Direct development: juvenile resembles adult.
    • Indirect development: larval stage undergoes metamorphosis (e.g. caterpillar → butterfly).
    • Embryogenesis proceeds from zygote → blastula → gastrulation.

Early Embryology & Body Plans

Gastrulation & Digestive Tracts

  • Blastula invaginates; the first opening = blastopore.
  • If cells migrate through to form a second opening ➔ one-way digestive tract.
  • If not ➔ two-way (single-opening) digestive tract (e.g. sponges, cnidarians).

Germ Layers & Derivatives

  • Ectoderm: epidermis, nervous system, sensory epithelia (eyes/ears/nose), tooth enamel.
  • Mesoderm: muscle, bone, blood, connective tissue, urogenital organs, spleen, cortex of adrenal gland.
  • Endoderm: lining of digestive & respiratory tracts, pancreas, liver ducts, thyroid parenchyma, bladder, urethra.

Symmetry & Cephalization

  • Asymmetry (sponges).
  • Radial symmetry: body repeats around central axis (jellyfish, sea anemone, starfish).
    • Little/no cephalization; often sessile or free-floating.
  • Bilateral symmetry: left & right mirror images.
    • Body axes: Anterior (front), Posterior (rear), Dorsal (back), Ventral (belly).
    • Movement leads with anterior end ➔ natural selection concentrates sense organs & nerve cells there.
    • Progressive clustering of neurons forms ganglia; large ganglia = brain.

Phylum Porifera (Sponges)

  • Earliest multicellular animals; invertebrates; acoelomate; mostly marine.
  • Suspension/filter feeders; sessile adults.
  • Mostly asymmetric; a few radial.
  • No true tissues/organs; but several specialized cell types.

Classes

  1. Calcarea – CaCO$_3$ spicules; marine; small (<4 mm); dull colours.
  2. Hexactinellida – "glass sponges"; six-rayed silica spicules; deep marine.
  3. Demospongiae – ≈\approx 90 % of all sponges; colourful; include bath sponges; some freshwater species.

Anatomy & Cell Types

  • Pinacocytes: outer epidermal layer; thin, leathery, tightly packed.
  • Porocytes/Myocytes: tubular cells regulate water entry; can contract.
  • Choanocytes: flagellated collar cells line internal chambers; generate water current; capture food; may form gametes.
  • Mesohyl: gelatinous matrix between layers containing:
    • Mesenchyme: amorphous cell layer.
    • Archaeocytes (amoebocytes): totipotent; digest & transport nutrients; form gametes; can differentiate.
    • Sclerocytes: secrete mineral spicules.
    • Spongocytes: produce spongin fibres.
  • Spicules: CaCO$_3$ or silica skeletal elements supporting body.

Canal Systems

  • Asconoid (simple): water ➔ ostia ➔ spongocoel ➔ osculum.
  • Syconoid (advanced): body wall folded; radial canals increase surface area.
  • Leuconoid (complex): extensively branched chambers; largest sponges.

Reproduction

  • Asexual: budding or fragmentation; regeneration high (used in aquaculture).
  • Sexual: most hermaphroditic; choanocytes form sperm, archaeocytes form eggs; internal fertilization ➔ planktonic amphiblastula larva.

Importance

  • Provide habitat; filter & clarify seawater; historical bath-sponge industry (now largely synthetic).

Phylum Cnidaria

  • Acoelomates with radial symmetry; two germ layers (ectoderm & endoderm) separated by mesoglea.
  • Body forms:
    1. Polyp – sessile, cylindrical, tentacles up; can bud asexually.
    2. Medusa – free-swimming, bell-shaped, tentacles down; sexual stage.
  • Gastrovascular cavity: single opening for digestion, circulation, respiration, excretion.
  • Primitive nerve net; no centralized brain.
  • Specialised stinging cells: cnidocytes containing nematocysts (barbed, toxin-loaded capsules). Trigger ➔ harpoon ejects into prey/predator.

Classes & Examples

  • Hydrozoa – hydra, Portuguese man-o-war; polyps and medusae.
  • Scyphozoa – true jellyfish (moon jelly, purple jelly).
  • Cubozoa – box jellies, sea wasps; potent venom.
  • Anthozoa – sea anemones, corals; only polyp stage.

Life Cycles

  • Jellyfish: polyp buds ➔ ephyrae ➔ adult medusae; medusae release gametes ➔ planula larva ➔ polyp.
  • Sea anemones: asexual pedal laceration/budding; sexual spawning; competing colonies create “no-man’s-land” zones.

Ecological Roles & Issues

  • Form coral reefs (calcium carbonate exoskeletons) hosting vast biodiversity.
  • Symbiosis with zooxanthellae algae (photosynthesis supplies nutrients).
  • Coral bleaching triggers: ↑ water temperature, pollution/run-off, pH change, UV, low tides exposing coral to air.
  • If coral dies, dependent ecosystems collapse.
  • Filter water, provide shelter, mutualisms (clownfish/anemone).

Phylum Platyhelminthes (Flatworms)

  • Acoelomate, unsegmented, dorsoventrally flattened.
  • Bilateral symmetry; some cephalization; many species (≈14 500) marine & freshwater.
  • Gas exchange & nutrient distribution via diffusion (no circulatory/respiratory system).
  • Classes: Turbellaria (free-living), Trematoda (flukes), Cestoda (tapeworms).

Free-Living Planarians (Class Turbellaria)

  • Size 3–12 mm; glide on cilia & mucus.
  • Eyespots (ocelli) sense light; auricles sense water currents.
  • Feeding: muscular pharynx protrudes to ingest prey/detritus; extracellular digestion begins in pharynx, completed intracellularly; branched gut distributes nutrients.
  • Nervous system: cerebral ganglion (primitive brain), two ventral nerve cords; ladder-like.
  • Excretory: protonephridia with flame cells; osmotic regulation expels excess water (important because tissue solute concentration > freshwater).
  • Reproduction:
    • Sexual: individuals exchange sperm (internal fertilization); eggs laid in capsules; zygotes hatch as miniatures.
    • Asexual: remarkable regeneration via mitosis; smallest regenerating piece recorded = 1279\tfrac{1}{279} of a worm (≈10 000 cells).

Parasitic Tapeworms (Class Cestoda)

  • Adapted to intestinal environment; lack digestive tract ➔ absorb nutrients through tegument.
  • Scolex with hooks/suckers, followed by proglottids packed with reproductive organs.
  • Pork tapeworm (Taenia solium) up to 2 m; beef tapeworm (T. saginata) similar.
  • Symptoms: abdominal discomfort, diarrhea, appetite loss.
  • Treatment: anthelmintic drugs dissolve worm.

Phylum Nematoda (Roundworms)

  • Pseudocoelomate; complete one-way digestive tract; unsegmented.
  • Thick collagen cuticle resists desiccation; must moult to grow.
  • Only longitudinal muscles ➔ whip-like thrashing.
  • Distinct sexes (dioecious); internal fertilization; reproductive organs elaborate; waste excreted via pore.
  • No circulatory or respiratory systems; rely on pseudocoelomic fluid & diffusion.

Ecological Roles

  • Abundant decomposers; recycle bacteria, fungi & detritus → soil nutrients.
  • Crucial in global energy & matter cycles.

Human & Agricultural Parasites

  • Whipworms (Trichuris) – large intestine; trichinosis.
  • Hookworms – small intestine; feed on blood.
  • Pinworms – colon; males 1–4 mm, females 8–13 mm.
  • Filarids – transmitted by mosquitoes; block lymph nodes ➔ elephantiasis (limbs/genitals swell).
  • Soybean cyst nematode – forms cysts in roots; crop loss.
  • Pine wood nematode – destroys forest trees.
  • Ascaris lumbricoides – ascariasis; ~1 billion people infected.
    • Life cycle summary:
    1. Adults in small intestine; females lay ≈200 000 eggs/day.
    2. Eggs shed in feces; embryonate 18 days–weeks18\text{ days}–\text{weeks} depending on conditions.
    3. Ingested eggs hatch; larvae penetrate intestinal wall ➔ blood ➔ lungs.
    4. Mature in lungs 10–1410–14 days, ascend trachea, swallowed.
    5. Return to intestine, grow to adults; 2–32–3 months from ingestion to egg-laying.
    6. Adults may live 1–21–2 years.
    • Extreme infection example: 2-year-old girl found with 796 worms in ileum.

Vocabulary & Concepts Checklist (Test Outline)

  • Cell specialization, division of labour; herbivore–carnivore–detritus–filter–parasite.
  • Respiration, internal transport, hemoglobin.
  • Excretion; response; sessile vs motile movement.
  • Fertilization (internal/external); direct vs indirect development; metamorphosis; embryogenesis; blastula; blastopore; gastrulation.
  • One-way vs two-way digestive tracts.
  • Germ layers: ectoderm, mesoderm, endoderm.
  • Symmetry terms; cephalization; ganglia.
  • Porifera structures (spicules, osculum, mesohyl, choanocyte, etc.).
  • Cnidarian terms (polyp, medusa, mesoglea, nematocyst, coral bleaching).
  • Platyhelminthes key words (ocelli, pharynx, regeneration, tapeworm).
  • Nematode terms (cuticle, pseudocoelom, dioecious, hookworm, filarid, elephantiasis, ascariasis).