Kingdom Animalia Overview

The Kingdom Animalia comprises multicellular, eukaryotic organisms that are heterotrophic and typically possess specialized tissues. This study guide reviews the major characteristics used to classify animals and surveys the principal phyla and classes within the kingdom.

General Characteristics Used for Animal Classification

Body Plan

Animals are classified based on their body plan, which refers to the structural organization of their bodies.

• Sac Body Plan: The body consists of a single opening that serves as both mouth and anus, leading to a gut cavity. Example: Hydra.

• Tube-in-Tube Body Plan: The body has a digestive tract with two openings (mouth and anus), allowing for a more complex digestive process. Example: Earthworm.

Symmetry

Symmetry describes how body parts are arranged around a central axis.

• Asymmetry: No symmetry; body cannot be divided into mirror-image halves. Example: Sponges.

• Radial Symmetry: Body parts arranged around a central axis; can be divided into similar halves by multiple planes. Example: Sea stars.

• Bilateral Symmetry: Body can be divided into two mirror-image halves along one plane. Example: Turtle.

Germ Layers

Germ layers are the primary layers of cells that form during embryonic development and give rise to tissues and organs.

• Diploblastic: Two germ layers: ectoderm and endoderm. Example: Cnidarians.

• Triploblastic: Three germ layers: ectoderm, mesoderm, and endoderm. Example: Flatworms, Vertebrates.

Body Cavity Type

The body cavity (coelom) is a fluid-filled space that houses internal organs.

• Acoelomate: No body cavity between the digestive tract and body wall. Example: Flatworms.

• Pseudocoelomate: Body cavity present, but not completely lined with mesoderm ("false coelom"). Example: Roundworms.

• Coelomate: Body cavity completely lined with mesoderm. Example: Earthworms, Vertebrates.

Blastopore Fate

The blastopore is the first opening formed during embryonic development; its fate distinguishes two major animal groups.

• Protostome: Blastopore becomes the mouth. Example: Mollusks, Arthropods.

• Deuterostome: Blastopore becomes the anus. Example: Echinoderms, Chordates.

Segmentation

Segmentation refers to the division of the body into repetitive segments.

• Non-segmented: Body is not divided into segments. Example: Flatworms.

• Segmented: Body is divided into repeated segments, often with duplication of organs. Example: Annelids, Arthropods.

Major Animal Phyla and Classes

Phylum Echinodermata

Echinoderms are marine animals characterized by a water vascular system, radial symmetry (in adults), and a calcareous endoskeleton.

• Class Crinoidea: Feather stars and sea lilies; most primitive echinoderms.

• Class Asteroidea: Sea stars; star-shaped body, tube feet with suckers, capable of arm regeneration.

• Class Ophiuroidea: Brittle stars; slender, flexible arms, no tube feet with suckers.

• Class Holothuroidea: Sea cucumbers; elongated body, no arms, tube feet modified for feeding.

• Class Echinoidea: Sea urchins and sand dollars; spherical or flattened body, no arms, calcareous plates form a test.

Phylum Hemichordata

Hemichordates are marine deuterostomes with features similar to both echinoderms and chordates, such as pharyngeal gill slits and a dorsal nerve cord. Example: Acorn worms.

Phylum Chordata

Chordates possess a notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail, and segmented muscles at some stage of development.

• Subphylum Urochordata: Tunicates and sea squirts; adults are sessile, sac-like body.

• Subphylum Cephalochordata: Lancelets; retain chordate features throughout life, segmented muscles.

• Subphylum Craniata (Vertebrata): Vertebrates; notochord replaced by vertebral column during development.

Major Vertebrate Groups

Vertebrates are classified into several classes based on skeletal structure, reproduction, and other features.

• Class Myxini: Hagfish; jawless, cartilaginous skeleton, produce slime.

• Class Cephalospidomorphi (Hyperoartia): Lampreys; jawless, cartilaginous skeleton.

• Class Chondrichthyes: Cartilaginous fish (sharks, skates, rays); lack swim bladder, internal fertilization, oviparous or ovoviviparous.

• Class Osteichthyes: Bony fish; possess swim bladder, operculum, external fertilization.

  • Subclass Sarcopterygii: Lobe-finned fish.

  • Subclass Actinopterygii: Ray-finned fish (e.g., goldfish, bass).

• Class Amphibia: Amphibians; moist skin, external fertilization, metamorphosis.

  • Order Anura: Frogs and toads; undergo metamorphosis.

  • Order Urodela: Salamanders; paedomorphosis common.

  • Order Apoda: Caecilians; limbless, worm-like.

• Class Reptilia: Reptiles; amniotic egg, internal fertilization, keratinized skin.

  • Order Chelonia: Turtles and tortoises.

  • Order Rhychocephalia: Tuatara; ancient lineage.

  • Order Squamata: Lizards and snakes.

  • Order Crocodilia: Crocodiles and alligators; four-chambered heart.

• Class Aves: Birds; feathers, endothermy, parental care, amniotic egg.

• Class Mammalia: Mammals; hair, mammary glands, differentiated teeth, endothermy.

  • Monotremes: Egg-laying mammals (platypus, echidna).

  • Marsupials: Pouched mammals (kangaroo, opossum).

  • Placentals: Advanced mammals; embryo nourished via placenta (e.g., primates, rodents, whales).

Extraembryonic Membranes in Amniotes

Amniotes (reptiles, birds, mammals) possess extraembryonic membranes that protect and nourish the embryo.

• Amnion: Encloses the amniotic fluid, cushioning the embryo.

• Chorion: Facilitates gas exchange.

• Allantois: Stores waste and aids in gas exchange.

• Yolk sac: Provides nutrients to the developing embryo.

Comparative Table: Animal Body Plans and Features

Key Terms and Definitions

• Notochord: Flexible rod providing support in chordate embryos.

• Coelom: Fluid-filled body cavity lined by mesoderm.

• Blastopore: First opening in the developing embryo.

• Metamorphosis: Transformation from larval to adult form.

• Amniotic Egg: Egg with specialized membranes for terrestrial development.

Summary and Study Tips

• Understand the differences between protostomes and deuterostomes.

• Be able to classify animals based on body plan, symmetry, germ layers, and body cavity type.

• Know examples and key features of major animal phyla and classes.

• Review the function and significance of extraembryonic membranes in amniotes.

Additional info: Some details and examples have been inferred and expanded for completeness and clarity.