General Zoology: Animal Symmetry

Animal Symmetry

• Radial symmetry allows division into similar halves by multiple planes along the longitudinal axis; found in sponges, jellyfish, and sea urchins.
• Biradial symmetry permits division into mirrored halves using only two planes; seen in comb jellies (Ctenophora).
• Radial and biradial symmetry is common in sessile, freely floating, or weakly swimming animals; notable groups include Cnidaria and Ctenophora, with some echinoderms exhibiting secondary radial symmetry in adulthood.
• Bilateral symmetry enables division into two mirrored halves along a sagittal plane; sea anemones and jellyfishes are examples of Radial animals exhibiting bilateral characteristics.
• Triploblastic animals possess three germ layers: ectoderm, endoderm, and mesoderm, typical of Bilateria.

Body Cavities and Coelom

• A body cavity refers to an internal space, primarily dominated by the digestive tract or gut cavity.
• The coelom is a fluid-filled space located between the gut and outer body wall, providing cushioning and aiding locomotion in some species.
• Sponges do not possess body cavities.

Homeostasis

• Homeostasis maintains a stable and normal internal environment, crucial for organism survival; any deviation prompts negative feedback mechanisms.
• Internal changes arise from two main sources: replenishment and expulsion of materials, and responses to external environmental changes.
• Cellular homeostasis is upheld through cooperation among all body systems.

Asexual Reproduction

• Asexual reproduction generates new individuals (clones) without gamete fusion, predominantly through mitotic cell division.
• Offspring from asexual reproduction typically share the same genotype unless mutations occur.
• Common methods include:
  • Binary Fission: Division into two or more parts, frequent in bacteria and protozoa.
  • Budding: Outgrowth from a parent, common in yeast and cnidarians (e.g., jellyfish, corals, hydras).
  • Fragmentation & Regeneration: Body division into pieces followed by regrowth, typical in planarians and echinoderms.

Advantages and Disadvantages of Asexual Reproduction

• Benefits include:
  • High offspring abundance
  • Fast reproduction rates
  • Elimination of courtship rituals
  • Lower energy requirements
  • Favorable conditions for colonization
• Drawbacks include:
  • Reduced genetic variability
  • Slower evolutionary progress
  • Increased vulnerability to diseases
  • Sensitivity to environmental changes

Sexual Reproduction

• Involves the formation of new individuals from gametes, typically through bisexual reproduction with two parent organisms.
• Less common forms include hermaphroditism (individuals possessing both male and female reproductive organs) and parthenogenesis (development of embryos without fertilization).