Introduction to Invertebrates: Major Animal Phyla and Their Characteristics

Chapter 33: An Introduction to Invertebrates

Sponges: Basal Animals Without Tissues

Sponges represent the most basal lineage of animals, meaning they are among the earliest and simplest forms of animal life. They lack true tissues and organs, distinguishing them from most other animal groups.

• Basal animals: Sponges are considered basal because they diverged early in animal evolution.

• Lack of tissues: Unlike most animals, sponges do not possess true tissues or organs.

Phylum Porifera: The Sponges

• Phylum: Sponges belong to the phylum Porifera.

• Characteristics:

  • Asymmetrical body plan

  • Filter feeders: Water flows through their porous bodies, and specialized cells called choanocytes capture food particles.

  • Skeleton composed of spicules or spongin fibers

Phylum Cnidaria: Key Characteristics

Cnidarians are a diverse group of mostly marine animals known for their specialized stinging cells.

• Radial symmetry

• Diploblastic: Two tissue layers (ectoderm and endoderm)

• Gastrovascular cavity: A central digestive compartment with a single opening

• Cnidocytes: Specialized stinging cells containing nematocysts for prey capture and defense

• Examples: Jellyfish, sea anemones, corals, hydras

Phylum with the Widest Range of Animal Body Forms: Phylum Mollusca

• Triploblastic: Molluscs have three germ layers (ectoderm, mesoderm, endoderm).

• Body cavity: They possess a true coelom (body cavity completely lined by mesoderm).

• Special structures:

  • Radula: A feeding organ found in many molluscs, used to scrape food.

  • Trochophore larva: A ciliated larval stage present in some molluscs.

• Characteristics:

  • Soft-bodied, often protected by a hard shell

  • Muscular foot for movement

  • Visceral mass containing internal organs

  • Mantle that secretes the shell

Lophophorates: Bryozoans and Brachiopods

Lophophorates are united by the presence of a lophophore, a crown of ciliated tentacles used for feeding.

• Bryozoans: Colonial animals that resemble mosses; each individual is called a zooid.

• Brachiopods: Marine animals with two shells (valves) on the upper and lower surfaces, unlike bivalve molluscs.

Phylum Mollusca: General Features

• Examples: Snails, clams, squids, octopuses

• Protection: Most have a calcium carbonate shell secreted by the mantle.

• Three main body parts:

  • Muscular foot

  • Visceral mass

  • Mantle

Major Classes of Mollusca

• Polyplacophora (Chitons): Oval-shaped, shell with eight plates; cling to rocks.

• Gastropoda: Snails and slugs; single, spiraled shell (or none in slugs); undergo torsion during development.

• Bivalvia: Clams, oysters, mussels, scallops; two-part hinged shell; filter feeders.

• Cephalopoda: Squids, octopuses, cuttlefish, nautiluses; well-developed nervous system; foot modified into tentacles; active predators.

Phylum Annelida: Segmented Worms

• Characteristics:

  • Body segmented into repeated units

  • True coelom

  • Closed circulatory system

  • Examples: Earthworms, leeches, polychaetes

Phylum with the Most Species-Rich Animal Group: Ecdysozoa

• Includes: Nematoda and Arthropoda

• Key feature: Shedding of the cuticle (ecdysis)

Phylum Nematoda: Roundworms

• Characteristics:

  • Unsegmented, cylindrical bodies

  • Pseudocoelomate

  • Complete digestive tract

  • Covered by a tough cuticle that is periodically shed

  • Many are free-living; some are important parasites (e.g., Ascaris, Trichinella)

Phylum Arthropoda: The Most Diverse Animal Phylum

• Cambrian explosion: Arthropods diversified greatly during this period (~535-525 million years ago).

• Habitat diversity: Found in nearly all environments on Earth.

• Key features:

  • Segmented body (head, thorax, abdomen)

  • Hard exoskeleton made of chitin

  • Jointed appendages

Benefits of the Exoskeleton in Arthropods

• Protection: Shields against predators and environmental hazards.

• Support: Provides structural support for muscle attachment and movement.

• Prevents desiccation: Reduces water loss in terrestrial environments.

Major Lineages of Arthropods

Lineage	Examples	Key Characteristics
Chelicerates	Spiders, scorpions, horseshoe crabs	Chelicerae (fang-like mouthparts), two body segments, no antennae
Myriapods	Centipedes, millipedes	Many body segments, one or two pairs of legs per segment
Pancrustaceans	Crustaceans (crabs, lobsters, shrimp), insects	Two pairs of antennae (crustaceans), three-part body (insects), wings (insects)

Metamorphosis in Arthropods

• Complete metamorphosis: Life cycle includes four distinct stages: egg, larva, pupa, adult (e.g., butterflies, beetles).

• Incomplete metamorphosis: Juveniles (nymphs) resemble adults but lack fully developed wings and reproductive organs; no pupal stage (e.g., grasshoppers, cockroaches).

Echinoderms and Chordates: Phylum Deuterostomia

• Phylum: Both belong to the superphylum Deuterostomia.

• Echinoderms: Marine animals with radial symmetry as adults, water vascular system, tube feet (e.g., sea stars, sea urchins).

• Chordates: Animals with a notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail at some stage of development (e.g., vertebrates such as fish, amphibians, reptiles, birds, mammals).

Additional info: The above notes expand on the learning objectives by providing definitions, examples, and context for each phylum and major group discussed. Where the original content referenced 'lecture slides,' standard textbook knowledge has been used to fill in details.