Animal Kingdom Classification: Phyla, Embryology, and Vertebrate Groups

Posted on Oct 16, 2025 in Biology

Invertebrates: The Animal Kingdom (Part 1)

Animals are characterized by their ability to move (motility).

Stages of Animal Embryonic Development

The formation of tissues and organs begins with three primary germ layers:

  • Endoderm: The inner layer, which forms the digestive organs.
  • Ectoderm: The external layer, which forms the nervous system and external coatings (skin).
  • Mesoderm: The intermediate layer, which forms muscles, bones, and circulatory systems.

Classification by Tissue and Symmetry

  • Parazoa (Subkingdom): Lack true tissues and definite shape (e.g., Porifera/Sponges).
  • Eumetazoa: Possess true tissues and a definite body shape.

Body Symmetry

  • Radiata: Radial symmetry (e.g., Cnidarians).
  • Bilateralia: Bilateral symmetry.

Body Cavity (Coelom) and Phylogenetic Relationships

  • Acoelomates: Lack a body cavity.
  • Pseudocoelomates: Body cavity located between the mesoderm and the endoderm.
  • Coelomates: Possess a true visceral cavity (coelom).

Coelomate Evolutionary Lines

  • Protostomes: The embryonic blastopore develops into the mouth.
  • Deuterostomes: The embryonic blastopore develops into the anus; the mouth forms from a new opening.

Major Invertebrate Phyla

Porifera (Sponges)

Sponges possess an osculum (the hole where water leaves). They have two layers of cells separated by mesenchyme.

Cnidarians (Coelenterates)

Characterized by Cnidoblasts (stinging cells). They exhibit two main body organization types:

  • Polyp: Cylindrical, sac-shaped body.
  • Medusa: Umbrella-shaped, with a cylindrical mouth (manubrium) hanging below.

Platyhelminthes (Flatworms)

Soft-bodied animals with bilateral symmetry and well-developed organs. Three classes:

  • Turbellaria: Free-living flatworms (e.g., Planarians).
  • Trematoda: Parasites (e.g., Liver Fluke).
  • Cestoda: Tape-like form (e.g., Tapeworms).

Nematoda (Roundworms)

Cylindrical and thin-bodied. Many are significant parasites:

  • Trichinella spiralis: Transmitted via pork, causing degeneration of muscle fibers.
  • Ascaris: Common human parasite (intestinal worms).

Annelida (Segmented Worms)

Coelomates with bodies divided into rings called metameres. The body is covered with epidermal cells and underlying muscle layers that allow movement. Setae (chaetae) are used for locomotion.

Classes of Annelids:

  • Polychaeta: Marine worms.
  • Oligochaeta: Earthworms.
  • Hirudinea: Leeches.

Mollusca (Mollusks)

Typically live in aquatic environments. Features include:

  • Unsegmented and soft body.
  • Reduced body cavity.
  • Presence of a muscular foot.
  • Organs contained within the visceral mass.

Classes:

  • Gastropoda: Snails.
  • Bivalvia: Body enclosed in two shells (valves).
  • Cephalopoda: Squid.

Echinodermata

Possess a tubular hydraulic system called the ambulacral system.

Arthropoda (Arthropods)

Characteristics:

  • Body segmented into rings.
  • Hard, rigid exoskeleton.
  • The exoskeleton must be shed (molting) periodically to allow the animal to grow.
  • Jointed appendages, usually two in each segment.
  • Specialized sense organs.

Classes:

  • Arachnida: Body divided into cephalothorax and abdomen (Spiders, Scorpions, Mites).
  • Crustacea: Many micro-crustaceans are part of zooplankton. Larger crustaceans often have five or more pairs of legs.
  • Insecta:
    • Exoskeleton provides waterproofing.
    • Small size and ability to fly.
    • High adaptability and large numbers of offspring.
    • Undergo metamorphosis.
  • Myriapoda: Centipedes and Millipedes.

Vertebrates: The Animal Kingdom (Part 2)

Phylum Chordata

Chordates exhibit three defining characteristics:

  • Notochord: A skeletal axis present in the embryo.
  • Longitudinal Nerve Tube: A dorsal hollow nerve cord.
  • Pharyngeal Slits: Gill slits in the throat region.

Subphylum Vertebrata

The body is divided into three parts: head, trunk, and tail. Most vertebrates possess a cloaca. Vertebrates are divided into Fish, Amphibians, Reptiles, Birds, and Mammals.

Fish (Pisces)

Divided into two large groups: Bony Fish and Cartilaginous Fish.

  • Bony Fish (Osteichthyes): Skin covered with overlapping scales. Possess paired fins (Pectoral and Ventral) and unpaired fins (Dorsal, Caudal).
  • Cartilaginous Fish (Chondrichthyes): Formed by sharks and rays. Body covered with denticles (placoid scales). Possess rigid fins and teeth inserted into the mouth.
  • Cyclostomes: The most primitive vertebrates (Jawless fish).

Amphibians

Body organization includes a head, trunk, and two pairs of limbs (chiridia). They are divided into two groups:

  • Anura: Short trunk, lacking a tail in the adult state.
  • Caudata: Possess a tail in adulthood.

Reptiles

The first vertebrates fully adapted to the terrestrial environment.

Groups of Reptiles:

  • Testudines: Turtles and tortoises (aquatic and terrestrial).
  • Crocodylia: Crocodiles and alligators.
  • Squamata (Lizards): Lizards, geckos, etc.
  • Squamata (Ophidians): Snakes.

Birds (Aves)

Key anatomical features:

  • Remiges: Flight feathers.
  • Coverts: Feathers that cover the body.
  • Down: Feathers used for insulation.
  • Scales: Cover the legs and feet.
  • Beak: Specialized mouth structure.

Mammals (Mammalia)

Types of Mammals:

  • Monotremes: Egg-laying mammals (e.g., Platypus).
  • Marsupials: Embryo develops partially, then finishes development in a pouch.
  • Placentals: Embryo develops fully within the uterus.
Locomotion Posture
  • Plantigrade: Support on the entire sole of the foot (e.g., Bears).
  • Digitigrade: Support only on the toes/fingers (e.g., Cats).
  • Unguligrade: Support on the hooves (e.g., Horses).