Animal Diversity and Phylogeny

Overview of Animal Phylogeny

The animal kingdom, Animalia, is a diverse group of multicellular, eukaryotic organisms. All animals share a common ancestor and are classified based on evolutionary relationships, body plans, and developmental characteristics. The major groups within Animalia include Porifera, Ctenophora, Cnidaria, Lophotrochozoa, Ecdysozoa, and Deuterostomia.

• Poriferans (sponges) are the sister group to all other animals.

• Eumetazoans possess true tissue layers.

• Most animals are Bilaterians, characterized by bilateral symmetry and three tissue layers.

• Bilaterians include Deuterostomia, Lophotrochozoa, and Ecdysozoa.

Key Features Unifying Animal Kingdom

Animals are defined by several unifying features:

• Multicellularity: Composed of multiple cells with specialized functions.

• Heterotrophy: Obtain energy by consuming other organisms.

• Motility: Most animals can move at some stage of their life cycle.

• Development from a blastula: Unique embryonic stage.

• Absence of cell walls: Unlike plants and fungi.

Animals belong to the supergroup Opisthokonta within Eukaryotes, which also includes fungi.

Major Invertebrate Phyla

Porifera (Sponges)

Porifera are simple, aquatic animals lacking true tissues and organs. They are asymmetrical and have a porous body structure.

• Tissues: None; cellular level of organization.

• Symmetry: Asymmetrical.

• Digestive system: None; filter feeders.

• Example: Sponges.

Ctenophora (Comb Jellies)

Ctenophores are marine animals characterized by rows of ciliary plates (ctenes) used for locomotion. They possess a simple digestive system and are often bioluminescent.

• Tissues: True tissues.

• Symmetry: Radial.

• Digestive system: Complete digestive tract.

• Example: Sea gooseberry (Pleurobrachia pileus).

Cnidaria

Cnidarians are aquatic animals with radial symmetry and specialized stinging cells called cnidocytes. They include jellyfish, corals, and sea anemones.

• Tissues: True tissues (diploblastic).

• Symmetry: Radial.

• Digestive system: Gastrovascular cavity (incomplete digestive system).

Lophotrochozoa

Lophotrochozoa is a clade of bilaterian animals that includes several phyla, such as Mollusca and Annelida. Members often have a lophophore (feeding structure) or trochophore larva.

• Symmetry: Bilateral.

• Coelom: True coelom (body cavity).

Phylum Mollusca

Mollusca is the second most diverse animal phylum, with over 100,000 species. Molluscs are characterized by a soft body, often protected by a hard shell.

• Symmetry: Bilateral.

• Special features: Foot (movement), mantle (shell secretion), visceral mass (internal organs).

• Major classes: Bivalvia, Gastropoda, Cephalopoda, Polyplacophora, Scaphopoda.

Ecdysozoa

Ecdysozoa is a clade of animals that grow by molting their exoskeleton (ecdysis). This group includes nematodes and arthropods.

• Defining feature: Ecdysis (molting).

• Symmetry: Bilateral.

• Coelom: True coelom.

Phylum Arthropoda

Arthropoda is the most diverse animal phylum, with over 1 million described species. Arthropods have jointed appendages and an exoskeleton made of chitin and protein.

• Symmetry: Bilateral.

• Coelom: True coelom.

• Defining characteristics: Jointed appendages, exoskeleton, segmented body.

• Major groups: Chelicerates (spiders, scorpions), Myriapods (centipedes, millipedes), Crustaceans (crabs, lobsters), Insects.

Deuterostomia

Deuterostomia is a clade of bilaterian animals characterized by deuterostome development (the blastopore becomes the anus). This group includes echinoderms and chordates.

Phylum Echinodermata

Echinoderms are marine animals with a unique water vascular system and tube feet. They exhibit radial symmetry as adults and have a coelom.

• Symmetry: Radial (adults), bilateral (larvae).

• Coelom: True coelom.

• Defining features: Water vascular system, tube feet.

Evolutionary Trends in Animalia

Trends and Derived Features

Evolutionary trends in animals include the development of tissues, symmetry, body cavities, and segmentation. These features are used to classify animals and understand their evolutionary relationships.

• Tissue development: From no tissues (Porifera) to true tissues (Eumetazoa).

• Symmetry: Asymmetry (Porifera), radial (Cnidaria, Ctenophora), bilateral (Bilateria).

• Body cavity: Acoelomate, pseudocoelomate, coelomate.

• Segmentation: Present in some groups (e.g., annelids, arthropods).

Animalia in the Tree of Life

Kingdom Animalia is placed within the domain Eukarya and supergroup Opisthokonta. Animals are closely related to fungi and are distinct from plants and protists.

Summary Table: Major Animal Groups

Review Questions

• What features unify the organisms within the Animal Kingdom?

• How are major animal groups classified in the phylogenetic tree?

• What are the evolutionary trends observed in Kingdom Animalia?

• How do the defining characteristics of each phylum relate to their evolutionary significance?