Kingdom Animalia: Evolution, Diversity, and Body Plans

Kingdom Animalia

Introduction

The animal kingdom, or Kingdom Animalia, encompasses a vast diversity of multicellular, heterotrophic eukaryotes. Animals are distinguished by their mode of nutrition, cellular structure, and developmental patterns. This chapter explores the defining features of animals, their evolutionary history, and the fundamental body plans that underlie animal diversity.

Animal Evolution and Diversity

What is an Animal?

• Animals are multicellular, heterotrophic eukaryotes that obtain nutrients primarily by ingestion (eating other organisms or organic material).

• Unlike plants and fungi, animals digest food within their bodies after ingestion.

• Animal cells lack cell walls and are instead supported by an extracellular matrix rich in collagen.

• Animals possess unique intercellular junctions (tight junctions, anchoring junctions, gap junctions) that facilitate communication and adhesion between cells.

• Specialized muscle cells enable movement, and nerve cells conduct electrical impulses.

Example: A snake ingesting a deer demonstrates the animal mode of nutrition—ingestion followed by internal digestion.

Animal Reproduction and Development

• Most animals are diploid and reproduce sexually.

• Male and female adults produce haploid gametes (sperm and egg) via meiosis.

• Fertilization produces a zygote, which divides by mitosis to form a multicellular blastula.

• One side of the blastula folds inward during gastrulation, forming a gastrula with three embryonic tissue layers:

  • Endoderm: Forms the lining of the digestive tract.

  • Ectoderm: Forms the outer covering and nervous system.

  • Mesoderm: Forms muscles and most internal organs.

Additional info: The internal sac formed during gastrulation becomes the digestive tract (alimentary canal).

Evolutionary History of Animals

The Ediacaran Biota

• The oldest generally accepted animal fossils are from the Ediacaran period (575–550 million years ago).

• Discovered in the Ediacara Hills, South Australia, the Ediacara biota consists of soft-bodied animals, some related to sponges and cnidarians, but many with no living relatives.

• Key fossils: Dickinsonia costata and Spriggina floundersi.

The Cambrian Explosion

• Occurred 535–525 million years ago.

• Marked by a rapid diversification of animal body plans and the appearance of most major animal phyla.

• Fossils from the Burgess Shale Formation in Canada provide evidence of this event.

• Notable Cambrian animals: Anomalocaris, Hallucigenia, early arthropods, and chordates.

Vertebrates and Invertebrates

• Of the ~35 animal phyla, only one includes vertebrates (animals with a backbone).

• Approximately 96% of animal species are invertebrates (animals without a backbone).

Animal Body Plans

Symmetry

• Symmetry is a key feature of animal body plans.

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

• Bilateral symmetry: Body has right and left halves that are mirror images (e.g., lobster).

Tissues and Body Cavities

• Animals may have two (diploblastic) or three (triploblastic) embryonic tissue layers.

• Triploblastic animals may possess a body cavity (coelom): a fluid-filled space between the digestive tract and body wall.

• Functions of the body cavity:

  • Cushions internal organs

  • Allows organs to grow and move independently of the body wall

  • Forms a hydrostatic skeleton in soft-bodied animals

• True coelom: Completely lined by mesoderm-derived tissue.

• Pseudocoelom: Not completely lined by mesoderm-derived tissue.

Embryonic Development Patterns

• During gastrulation, the first opening (blastopore) can become the mouth (protostome development) or the anus (deuterostome development).

• Animals are classified based on their embryonic development and body cavity type.

Major Animal Groups and Body Plan Classification

Group	Tissues	Symmetry	Body Cavity	Examples
Sponges	No true tissues	None	None	Sponges
Cnidarians	True tissues	Radial	None	Jellyfish, sea anemones
Flatworms	True tissues	Bilateral	Acoelomate	Planarians
Nematodes	True tissues	Bilateral	Pseudocoelomate	Roundworms
Annelids, Arthropods, Molluscs, Echinoderms, Chordates	True tissues	Bilateral	Coelomate	Earthworms, insects, snails, sea stars, vertebrates

Additional info: This table summarizes the major animal groups based on tissue organization, symmetry, and body cavity type, as depicted in the evolutionary tree.

Summary

• Animals are defined by their multicellularity, heterotrophy, lack of cell walls, and unique cellular junctions.

• Animal evolution began over 550 million years ago, with major diversification during the Cambrian explosion.

• Body plans are classified by symmetry, tissue layers, and the presence/type of body cavity.

• Most animal phyla are invertebrates; vertebrates represent a single phylum.