Animal Diversity: Introduction and Survey of Major Invertebrate Groups

Animal Diversity: Overview

Introduction to Metazoans

Metazoans, or animals, are a diverse group of eukaryotic, multicellular organisms that are primarily chemoheterotrophic. This section introduces the defining characteristics of animals and their evolutionary relationships.

• Eukaryotic: Cells contain a nucleus and membrane-bound organelles.

• Multicellular: Composed of multiple cells with specialized functions.

• Chemoheterotrophic: Obtain energy and carbon by ingesting other organisms.

• Diploid Life Cycle: Most life stages are diploid; only gametes are haploid.

• No Cell Walls: Animals lack cell walls, using extracellular structural proteins (especially collagen).

• Food Ingestion: Animals ingest food and digest it internally.

Basic Animal Body Plans

Animal body plans are categorized by symmetry, tissue organization, and developmental patterns.

• Symmetry:

  • Radial symmetry: Body parts arranged around a central axis (e.g., cnidarians).

  • Bilateral symmetry: Body has distinct left and right sides (e.g., most animals).

  • Asymmetry: No symmetry (e.g., sponges).

• Tissue Layers:

  • Diploblasts: Two germ layers (ectoderm and endoderm).

  • Triploblasts: Three germ layers (ectoderm, mesoderm, endoderm).

• Body Cavities:

  • Acoelomate: No body cavity.

  • Coelomate: True body cavity within mesoderm.

  • Pseudocoelomate: Body cavity not fully lined by mesoderm.

Animal Phylogeny & Classification Features

Animals are classified based on several key features:

• Presence of tissues

• Body symmetry

• Body segmentation

• Molting exoskeleton

• Embryonic tissue layers

• Digestive openings

• Mouth formation (protostome vs. deuterostome)

• Presence of body cavity

Survey of Major Invertebrate Groups

What is an "Invertebrate"?

Invertebrates are animals that lack a vertebral column. While "vertebrates" are a well-defined group, "invertebrates" encompass a vast diversity of animal phyla.

• Includes sponges, cnidarians, flatworms, molluscs, annelids, nematodes, arthropods, echinoderms, and more.

• Some scientists avoid the term due to its broad and non-monophyletic nature.

Phylum Porifera (Sponges)

Sponges are the simplest animals, lacking true tissues and organs. They closely resemble colonial protists.

• No true tissues: Composed of specialized cells (choanocytes and amoebocytes).

• Choanocytes: Flagellated cells that create water flow and capture food particles; resemble choanoflagellates.

• Body Plan: Asymmetric.

• No nervous system: Individual cells may have nerve cell machinery.

• Reproduction: Most are hermaphroditic (often sequentially).

• Adults are sessile; larvae are motile.

Porifera Table

Feature	Porifera
Symmetry	None (asymmetric)
Tissues	No
Body Cavity	None
Digestive Openings	None (filter feeding)
Examples	Sponges

Phylum Cnidaria

Cnidarians are the first eumetazoans, possessing true tissues but only two germ layers (diploblastic). They include jellyfish, corals, and hydra.

• Body Forms: Polyp and medusa forms; some species alternate between forms.

• Symmetry: Radial.

• Nervous System: Simple, diffuse neural network (no brain).

• Digestive System: Single opening (gastrovascular cavity).

• Cnidocytes: Specialized stinging cells; three types (stinging, entangling, gripping).

• Corals: Build calcium carbonate skeletons.

Cnidaria Table

Feature	Cnidaria
Symmetry	Radial
Tissues	Yes (diploblastic)
Body Cavity	Gastrovascular cavity
Digestive Openings	One
Examples	Jellyfish, corals, hydra

Hydra: A Model Cnidarian

Hydra are freshwater cnidarians notable for their regenerative abilities and use in research.

• Named after Greek mythology.

• Exist only in polyp form.

• "Immortal" animals: Can regenerate lost parts; valuable for regenerative medicine studies.

• Frankenhydra: Experimental rearrangement of hydra tissues demonstrates cellular plasticity.

Phylum Ctenophora (Comb Jellies)

Ctenophores are another early-diverging animal group, exhibiting biradial symmetry and a rudimentary nervous system.

• Biradial symmetry.

• More cell types than sponges.

• Rudimentary nervous system.

Lophotrochozoans: Flatworms, Molluscs, Annelids

Lophotrochozoa is a major clade of bilaterian animals, including flatworms, molluscs, and annelids.

Flatworms (Platyhelminthes)

• Bilateral symmetry.

• Triploblastic: Three germ layers.

• Acoelomate: No body cavity.

• One digestive opening.

• Examples: Planaria (regenerative), trematodes (flukes), tapeworms (parasitic).

Molluscs

• Complete nervous system.

• Varied body plans: Includes snails, clams, squids, and sea slugs (nudibranchs).

Annelids (Segmented Worms)

• Segmented body plan: Repeated segments with similar tissues and organs.

• Closed circulatory system: Both dorsal and ventral vessels.

• Specialized digestive tract: Not segmented.

• Examples: Earthworms, leeches.

What is a "Worm"?

"Worm" is a general term for animals with elongated, soft bodies. It is not a taxonomic group but includes flatworms, segmented worms (annelids), and roundworms (nematodes).

Summary Table: Major Animal Features

Phylum	Symmetry	Tissues	Body Cavity	Digestive Openings	Examples
Porifera	None	No	None	None	Sponges
Cnidaria	Radial	Yes (2 layers)	Gastrovascular	One	Jellyfish, corals, hydra
Flatworms	Bilateral	Yes (3 layers)	None	One	Planaria, tapeworms
Molluscs	Bilateral	Yes (3 layers)	Coelom	Two	Snails, clams, squids
Annelids	Bilateral	Yes (3 layers)	Coelom	Two	Earthworms, leeches

Additional info: The notes provide a foundational overview of animal diversity, focusing on major invertebrate groups and their evolutionary relationships. This content is highly relevant for introductory biology or zoology courses, and provides essential context for understanding animal classification and body plans.