Chapter 18

Introduction to Invertebrate Diversity

• Insects demonstrate vast diversity in:

  • Shapes and sizes

  • Habitats

  • Diets

  • Mating habits

  • Other characteristics

• Over one million species of insects, making up nearly three-quarters of all animal species.

Characteristics of Animals

• All animals are:

  • Eukaryotic

  • Multicellular heterotrophs

  • Cells lack cell walls

• Animals ingest food as part of their nutrition.

• Unique life cycles and embryonic development distinguish animals from other organisms.

History of Animal Diversification

• First animal fossils date back to the late Ediacaran period (~560 million years ago).

• Rapid diversification occurred during the Cambrian explosion (535-525 million years ago); many had hard body parts (shells, spikes).

Body Plans of Animals

• Characteristics defining animal body plans:

  • Symmetry (radial vs bilateral)

  • Presence of true tissues

  • Number of embryonic layers

  • Presence of a body cavity

  • Embryonic development details

Classification of Animals

• Approximately 35 animal phyla:

  • 96% are invertebrates (lack backbone)

  • Only one phylum (Chordata) contains vertebrates.

Phylum Porifera: Sponges

• Characteristics:

  • Simple, porous body structure

  • Typically lack body symmetry

  • Filter feeders (sieve food from water).

Phylum Cnidaria: Cnidarians

• Features:

  • Radial symmetry

  • Gastrovascular cavity for digestion

  • Two body forms: polyp (e.g., hydras) and medusa (e.g., jellies).

Phylum Platyhelminthes: Flatworms

• Simplest bilateral animals with:

  • No body cavity

  • Gastrovascular cavity

  • Examples include planarians, flukes, and tapeworms (some parasitic).

Phylum Nematoda: Nematodes

• Characteristics:

  • Bilateral symmetry

  • Body cavity and complete digestive tract (mouth and anus)

  • Some are decomposers; others are parasites.

Phylum Mollusca: Molluscs

• Diverse group including:

  • Muscular foot for locomotion

  • Visceral mass with internal organs

  • Mantle that may secrete a shell

  • Circulatory system; many use a radula to scrape food.

Phylum Annelida: Annelids

• Features:

  • Segmentation (repeated body parts)

  • Nervous system and closed circulatory system.

Phylum Arthropoda: Arthropods

• Over one million species:

  • Includes crayfish, lobsters, insects

  • Segmented body, jointed appendages, and hard exoskeleton.

  • Molting process (ecdysis) to grow larger.

Phylum Echinodermata: Echinoderms

• Characteristics:

  • Include sea stars and sea urchins

  • Radially symmetrical as adults

  • Unique water vascular system for movement using tube feet.

Phylum Chordata: Chordates

• Four defining features:

  • Dorsal, hollow nerve cord

  • Notochord (supportive rod)

  • Pharyngeal slits

  • Post-anal tail.

Evolutionary Success of Insects

• Insects represent 75% of all identified animal species; up to 30 million species estimated.

• Body features:

  • Head, thorax, abdomen, three pairs of legs, often wings.

• Life cycle includes metamorphosis:

  • Complete metamorphosis (larva, pupa, adult)

  • Incomplete metamorphosis (multiple molts without forming a pupa).

The Role of Homeotic Genes in Evolution

• Homeotic genes direct animal development; changes in their regulation led to diversity in segments and appendages among arthropods.

Importance of Invertebrate Diversity

• Invertebrates provide crucial ecosystem services and resources:

  • Freshwater mussels improve water quality

  • Reef-dwelling cone snails produce medicinal compounds

  • Pollination of flowering plants largely reliant on insects.

Conclusion

• Understanding animal characteristics, evolution, and the critical role of invertebrates is essential for appreciating biodiversity and its threats.