Invertebrate Animals: Structure, Function, and Evolutionary Diversity

Unifying Themes in Animal Biology

Structure and Function

The anatomy and morphology of animal cells, tissues, and organs are closely linked to their physiological functions. Understanding this relationship is fundamental to studying animal biology.

• Structure matches function: For example, the elongated shape of muscle cells facilitates contraction.

• Acquisition and manipulation of materials and energy: Animals have specialized systems for obtaining nutrients and energy from their environment.

• Homeostasis: Animals regulate their internal environment to maintain stable conditions necessary for survival.

• Adaptation and evolution: Animal taxa are related through descent with modification, leading to diverse forms and functions.

Major Concepts and Comparative Anatomy

Embryonic Development and Body Plans

Animal phyla are distinguished by their embryonic development and basic body plans.

• Diploblastic: Animals with two germ layers (ectoderm and endoderm), e.g., Cnidaria.

• Triploblastic: Animals with three germ layers (ectoderm, mesoderm, endoderm), e.g., most animal phyla.

• Acoelomates: No body cavity (coelom), e.g., Platyhelminthes.

• Pseudocoelomates: Body cavity not fully lined by mesoderm, e.g., Nematoda.

• Coelomates: True coelom fully lined by mesoderm, e.g., Annelida, Mollusca, Arthropoda.

• Symmetry: Asymmetry (e.g., Porifera), radial symmetry (e.g., Cnidaria), bilateral symmetry (most other phyla).

• Digestive systems: Gastrovascular cavity (single opening, e.g., Cnidaria), alimentary canal (tube within a tube, e.g., Annelida).

• Protostome vs. Deuterostome: Protostomes develop mouth first (e.g., Mollusca, Arthropoda), deuterostomes develop anus first (e.g., Echinodermata).

Cladograms and Evolutionary Relationships

Cladograms are diagrams that show evolutionary relationships based on shared derived characteristics.

• Interpretation: Identify common traits and evolutionary branching points.

• Creation: Use morphological and genetic data to construct cladograms.

Evolutionary Adaptiveness of Body Cavities and Segmentation

Body cavities and segmentation provide evolutionary advantages.

• Body cavities: Allow for organ development and movement independent of body wall.

• Segmentation: Facilitates specialization and redundancy in body regions (e.g., Annelida, Arthropoda).

Defining Characteristics of Major Invertebrate Phyla

Porifera (Sponges)

• Choanocytes: Flagellated cells that create water currents and capture food.

• Amoebocytes: Mobile cells involved in digestion and distribution of nutrients.

• Spicules: Structural elements made of silica or calcium carbonate.

• Spongin: Flexible protein fibers providing support.

Cnidaria (Jellyfish, Corals, Sea Anemones)

• Epidermis: Outer tissue layer.

• Gastrodermis: Inner tissue layer lining the gastrovascular cavity.

• Mesoglea: Gel-like layer between epidermis and gastrodermis.

• Cnidocytes: Specialized cells containing nematocysts for defense and prey capture.

• Gastrovascular cavity: Central cavity for digestion and circulation.

Platyhelminthes (Flatworms)

• Acoelomate: No body cavity.

• Simple organ systems: Includes basic nervous, digestive, and excretory systems.

Nematoda (Roundworms)

• Pseudocoelomate: Body cavity not fully lined by mesoderm.

• Complete digestive tract: Alimentary canal with mouth and anus.

Mollusca (Snails, Clams, Squid)

• Mantle: Tissue that secretes the shell.

• Radula: Tongue-like organ for feeding.

• Visceral mass: Contains internal organs.

• Muscular foot: Used for movement.

Annelida (Segmented Worms)

• Segmentation: Repeated body units.

• Coelomate: True body cavity.

Arthropoda (Insects, Crustaceans, Spiders)

• Exoskeleton: Hard outer covering made of chitin.

• Jointed appendages: Specialized for movement and feeding.

• Segmentation: Distinct body regions (head, thorax, abdomen).

Echinodermata (Sea Stars, Sea Urchins)

• Endoskeleton: Internal skeleton made of calcareous plates.

• Radial symmetry (adults): Body parts arranged around a central axis.

Comparative Body Systems Across Animal Phyla

Skeletomuscular Systems

• Hydrostatic skeleton: Fluid-filled cavity provides support (e.g., Annelida).

• Endoskeleton: Internal support structure (e.g., Echinodermata).

• Exoskeleton: External support and protection (e.g., Arthropoda).

Muscle types: Longitudinal and circular muscles enable movement.

Circulatory Systems

• Intracellular: Diffusion within cells (e.g., Porifera).

• Open circulatory system: Hemolymph bathes organs directly (e.g., Arthropoda).

• Closed circulatory system: Blood contained within vessels (e.g., Annelida).

Respiratory Systems

• Diffusion: Gas exchange across body surface (e.g., Platyhelminthes).

• Gills, tracheae, book lungs: Specialized structures in various phyla.

Digestive Systems

• Intracellular digestion: Occurs within cells (e.g., Porifera).

• Gastrovascular cavity: Single opening for ingestion and egestion (e.g., Cnidaria).

• Alimentary canal: Complete digestive tract with separate mouth and anus (e.g., Nematoda, Annelida).

Excretory Systems

• Intracellular: Simple diffusion (e.g., Porifera).

• Protonephridia (flame cells): Specialized excretory cells (e.g., Platyhelminthes).

• Metanephridia: Tubular excretory structures (e.g., Annelida).

• Malpighian tubules: Excretory organs in Arthropoda.

Nervous Systems

• Simple nerve nets: Found in Cnidaria.

• Centralized nervous systems: Found in more complex phyla (e.g., Arthropoda).

Reproductive Systems

• Sexual reproduction: External fertilization (e.g., aquatic animals), internal fertilization (e.g., terrestrial animals).

• Asexual reproduction: Budding, fission, fragmentation with regeneration.

Comparison of Major Animal Phyla

Phylum	Body Plan	Symmetry	Body Cavity	Digestive System	Skeleton	Circulatory System	Reproduction
Porifera	Cellular	Asymmetrical	None	Intracellular	Spicules/Spongin	None	Asexual/Sexual
Cnidaria	Tissue	Radial	None	Gastrovascular cavity	Hydrostatic	None	Asexual/Sexual
Platyhelminthes	Organ	Bilateral	Acoelomate	Gastrovascular cavity	Hydrostatic	None	Sexual
Nematoda	Organ	Bilateral	Pseudocoelomate	Alimentary canal	Hydrostatic	None	Sexual
Mollusca	Organ	Bilateral	Coelomate	Alimentary canal	Exoskeleton (shell)	Open	Sexual
Annelida	Organ	Bilateral	Coelomate	Alimentary canal	Hydrostatic	Closed	Sexual
Arthropoda	Organ	Bilateral	Coelomate	Alimentary canal	Exoskeleton	Open	Sexual
Echinodermata	Organ	Radial (adult)	Coelomate	Alimentary canal	Endoskeleton	Water vascular system	Sexual

Examples and Applications

• Example: The hydrostatic skeleton in earthworms (Annelida) allows for burrowing and movement through soil.

• Example: The exoskeleton in insects (Arthropoda) provides protection and support for terrestrial life.

• Example: Cnidocytes in jellyfish (Cnidaria) are used for defense and prey capture.

Additional info:

• Body segmentation is a key innovation in animal evolution, allowing for specialization of body regions and increased complexity.

• Cladograms are essential tools for understanding evolutionary relationships and can be constructed using both morphological and molecular data.

• Homeostasis is maintained through coordinated actions of multiple body systems, including nervous, endocrine, and excretory systems.