An Overview of Animal Diversity

Chapter 32: An Overview of Animal Diversity

Introduction to Animal Diversity

Animals are a diverse group of multicellular, heterotrophic eukaryotes that share a set of key characteristics. This chapter explores the defining features of animals, their evolutionary history, and the major body plans that distinguish animal phyla.

Defining Characteristics of Animals

Nutritional Mode

• Heterotrophy: Animals obtain energy and nutrients by ingesting other organisms, unlike plants (autotrophs) and fungi (which absorb nutrients after external digestion).

• Internal Digestion: Animals ingest food and digest it internally within specialized digestive systems.

Cell Structure and Specialization

• Multicellularity: Animals are composed of multiple cells lacking cell walls, supported instead by structural proteins such as collagen.

• Specialized Tissues: Unique to animals are nervous and muscle tissues, which enable rapid response and movement.

• Tissues: Groups of similar cells that act as functional units.

Reproduction and Development

• Sexual Reproduction: Most animals reproduce sexually, with the diploid stage dominating the life cycle.

• Cleavage: After fertilization, the zygote undergoes rapid cell divisions (cleavage) without growth, forming a blastula (hollow ball of cells).

• Gastrulation: The blastula undergoes gastrulation, forming a gastrula with embryonic tissue layers (germ layers).

• Larval Stages: Many animals have a larval stage that is morphologically and behaviorally distinct from the adult.

Genetic Control of Development

• Developmental Genes: All animals possess genes that regulate the expression of other genes during development.

• Hox Genes: Most animals share a unique family of regulatory genes (Hox genes) that control body plan and morphology.

Evolutionary History of Animals

Origin of Multicellular Animals

Morphological and molecular evidence indicates that animals evolved from choanoflagellate-like protists. The closest living relatives to animals are choanoflagellates, which share genes involved in cell adhesion and signaling with animals.

Major Eras in Animal Evolution

• Neoproterozoic Era (1 Billion–541 Million Years Ago): First macroscopic animal fossils (Ediacaran biota) appear; evidence of early predation.

• Paleozoic Era (541–252 Million Years Ago): Cambrian explosion leads to rapid diversification; first hard-bodied animals; colonization of land by arthropods and vertebrates.

• Mesozoic Era (252–66 Million Years Ago): Emergence of coral reefs, dinosaurs, mammals, and diversification of flowering plants and insects.

• Cenozoic Era (66 Million Years Ago–Present): Mammals diversify and increase in size; primate ancestors of humans appear.

Animal Body Plans

Symmetry

• Radial Symmetry: Body parts arranged around a central axis; typical of sessile or planktonic animals (e.g., cnidarians).

• Bilateral Symmetry: Body has right and left sides, dorsal (top) and ventral (bottom), anterior (head) and posterior (tail) ends; associated with active movement and cephalization (concentration of sensory organs at the head).

Tissues and Germ Layers

• Diploblastic Animals: Have two germ layers (ectoderm and endoderm); e.g., cnidarians.

• Triploblastic Animals: Have three germ layers (ectoderm, mesoderm, endoderm); includes all bilaterians.

Body Cavities

• Coelom: A true body cavity completely lined by mesoderm-derived tissue; cushions organs and allows independent movement of internal organs.

• Hemocoel: A body cavity formed between mesoderm and endoderm, filled with hemolymph (e.g., in molluscs and arthropods).

• Acoelomate: Animals lacking a body cavity; typically compact with tissues filling the space between gut and body wall.

Developmental Modes: Protostomes vs. Deuterostomes

• Cleavage: Protostomes exhibit spiral, determinate cleavage; deuterostomes have radial, indeterminate cleavage.

• Coelom Formation: In protostomes, the coelom forms from solid masses of mesoderm; in deuterostomes, it forms from folds of the archenteron.

• Fate of the Blastopore: In protostomes, the blastopore becomes the mouth; in deuterostomes, it becomes the anus.

Feature	Protostome Development	Deuterostome Development
Cleavage	Spiral, determinate	Radial, indeterminate
Coelom Formation	Solid masses of mesoderm split and form coelom	Folds of archenteron form coelom
Fate of Blastopore	Mouth develops from blastopore	Anus develops from blastopore

Animal Phylogeny

Major Clades and Relationships

• All animals share a common ancestor.

• Sponges are the sister group to all other animals.

• Eumetazoa is a clade of animals with true tissues.

• Most animal phyla belong to the clade Bilateria, which is divided into three major clades: Deuterostomia, Ecdysozoa, and Lophotrochozoa.

• Most bilaterians are invertebrates; only Chordata includes vertebrates.

Bilaterian Clades

• Deuterostomia: Includes hemichordates, echinoderms, and chordates (vertebrates and some invertebrates).

• Ecdysozoa: All members secrete an exoskeleton and undergo ecdysis (molting); includes nematodes and arthropods.

• Lophotrochozoa: Named for the presence of a lophophore (feeding structure) or trochophore larva in some members; includes molluscs, annelids, and ectoprocts.

Summary Table: Major Animal Clades

Clade	Key Features	Examples
Deuterostomia	Blastopore becomes anus; radial cleavage	Echinoderms, chordates
Ecdysozoa	Exoskeleton, ecdysis (molting)	Nematodes, arthropods
Lophotrochozoa	Lophophore or trochophore larva	Molluscs, annelids, ectoprocts

Conclusion

Animal diversity is the result of over half a billion years of evolution, with major innovations in body plans, tissues, and developmental modes. Understanding these features provides a framework for studying the vast array of animal life on Earth.