Invertebrates: Structure, Diversity, and Major Phyla (Chapter 33 Study Guide)

Invertebrates: Structure, Diversity, and Major Phyla

Overview of Invertebrates

Invertebrates comprise all animals except those in Subphylum Vertebrata. They represent a diverse array of body plans and evolutionary adaptations, making up the majority of animal phyla. Unlike vertebrates, invertebrates do not form a monophyletic group, meaning they do not all share a single common ancestor exclusive to them.

• Definition: Invertebrates are animals lacking a vertebral column (backbone).

• Monophyly: Invertebrates are a 'catch-all' group and are not monophyletic.

• Body Plan Variation: Key differences include cephalization (development of a head), segmentation, coelom (body cavity), and body symmetry (radial, bilateral, or none).

• Unique Features: Many phyla have specialized structures (e.g., the mantle in molluscs).

• Evolutionary History: All animal phyla have evolved for similar lengths of time but have adapted differently.

Phylum Porifera (Sponges)

Sponges are simple, sessile animals that closely resemble their colonial choanoflagellate ancestors. They lack true tissues and organs but have specialized cells.

• Body Plan: Sac-like body perforated with pores; no body symmetry.

• Tissues: Specialized cells, but no true tissues or organs.

• Locomotion: Adults are sessile; larvae are motile.

• Reproduction: Hermaphroditic; internal fertilization; free-swimming larvae.

• Gas Exchange: Diffusion through body wall.

• Nutrition: Filter feeders; flagellated choanocytes create water currents to capture food particles.

Phylum Cnidaria (Jellies, Hydras, Corals, Sea Anemones)

Cnidarians are radially symmetrical animals with specialized stinging cells. They have two main body forms: polyp and medusa.

• Body Plan: Sac with central digestive cavity; radial symmetry; oral and aboral ends.

• Polyp: Sessile, mouth and tentacles upward.

• Medusa: Free-swimming, mouth downward.

• Tissues: Diploblastic (ectoderm and endoderm); ectoderm forms epidermis, nerve net, and contractile fibers.

• Locomotion: Polyps are sessile; medusae move by contracting their bell.

• Gas Exchange: Diffusion through body wall.

• Nutrition: Incomplete digestive tract; predators using cnidocytes (stinging cells) and nematocysts to capture prey.

Phylum Platyhelminthes (Flatworms)

Flatworms are acoelomate, triploblastic animals with a flattened body, allowing for efficient gas exchange by diffusion.

• Body Plan: Flat, acoelomate; triploblastic (three germ layers).

• Digestive System: Incomplete; gastrovascular cavity with branching network.

• Classes:

  • Turbellaria: Free-living, mostly marine; some freshwater (e.g., planarians).

  • Monogenea: Parasitic, mainly on fish.

  • Trematoda: Parasitic flukes, often with complex life cycles involving multiple hosts.

  • Cestoda: Tapeworms; internal parasites lacking digestive tract, composed of proglottids.

• Cephalization: Present in some (e.g., planarians with eyespots).

• Reproduction: Many are hermaphroditic; complex life cycles in parasitic forms.

Phylum Syndermata (Rotifers)

Rotifers are tiny, multicellular animals with specialized organs and a complete digestive tract. They possess a pseudocoelom and a unique feeding structure called the wheel organ.

• Size: Microscopic to 0.2 mm.

• Body Cavity: Pseudocoelomate; fluid-filled cavity acts as a hydrostatic skeleton and circulates nutrients.

• Digestive System: Complete (mouth and anus).

• Feeding: Wheel organ (corona) with cilia creates water currents to draw in food.

• Reproduction: Usually parthenogenetic (females produce diploid eggs without fertilization); sexual reproduction occurs under stress, producing resistant zygotes.

Phylum Mollusca

Molluscs are soft-bodied animals, often with a hard shell. They have a muscular foot, visceral mass, mantle, and often a radula for feeding.

• Key Features:

  1. Muscular foot (movement)

  2. Visceral mass (contains organs)

  3. Mantle (secretes shell)

  4. Radula (rasping feeding organ, in most)

• Respiration: Most have gills; terrestrial forms use mantle as a lung.

• Cephalization: Well-developed in many classes.

Molluscan Class	Characteristics
Chitons	Oval-shaped; scrape algae from surfaces
Gastropods (snails, slugs)	Terrestrial or aquatic; torsion during development; radula for feeding; some are carnivorous
Bivalves (clams, oysters)	No radula or head; two-part shell; gills for gas exchange and feeding; sedentary or sessile
Cephalopods (squids, octopi, nautili)	Active predators; highly developed nervous system; reduced or absent shell; jet propulsion via siphon

Phylum Annelida (Segmented Worms)

Annelids are coelomate worms with segmented bodies. The coelom is divided by septa, and the hydrostatic skeleton aids in movement.

• Segmentation: Body divided into repeated segments.

• Support: Hydrostatic skeleton from fluid-filled coelom.

• Classes:

  • Sedentarian: Less mobile (earthworms, leeches, tube worms).

  • Errantians: More mobile, mostly aquatic; parapodia for movement and gas exchange.

Phylum Nematoda (Roundworms)

Nematodes are unsegmented, pseudocoelomate worms with a complete digestive tract. They are covered by a cuticle that is periodically shed.

• Body Plan: Unsegmented, cylindrical.

• Musculature: Only longitudinal muscles; movement is whip-like.

• Cuticle: Secreted and periodically shed (molting).

• Reproduction: Usually dioecious (separate sexes).

• Ecology: Free-living and parasitic species (e.g., cause of trichinosis in humans).

Phylum Arthropoda

Arthropods are the most diverse animal phylum, characterized by segmentation, jointed appendages, and a chitinous exoskeleton that is periodically molted.

• Key Features:

  1. Segmented body

  2. Exoskeleton (molted for growth)

  3. Jointed appendages

  4. High cephalization (well-developed sensory organs)

Arthropod Lineage	Characteristics
Chelicerata (spiders, scorpions, ticks, mites, horseshoe crabs)	Mostly terrestrial; two body segments (cephalothorax and abdomen); chelicerae (fang-like mouthparts)
Myriapoda: Diplopoda (millipedes)	Many segments; two pairs of legs per segment; detritivores
Myriapoda: Chilopoda (centipedes)	Many segments; one pair of legs per segment; carnivorous; venomous fangs
Pancrustaceans: Insects	Head, thorax, abdomen; three pairs of legs; usually two pairs of wings; tracheal system for gas exchange
Other Pancrustaceans (crustaceans: crabs, lobsters, shrimp, etc.)	Mostly marine; gills in larger species; diverse appendages; important planktonic forms (copepods)

• Ecological Importance of Insects:

  1. Pollination of flowering plants

  2. Key roles in food webs (predators and prey)

  3. Vectors for parasites and pathogens

Plankton vs. Nekton

• Plankton: Organisms that cannot swim against currents (e.g., copepods, unicellular algae).

• Nekton: Larger animals capable of swimming against currents (e.g., fish, turtles).

Phylum Echinodermata

Echinoderms are slow-moving or sessile deuterostomes with a unique water vascular system and radial symmetry as adults. They include sea stars, sea cucumbers, sand dollars, and others.

• Symmetry: Bilateral as larvae; radial as adults.

• Skeleton: Endoskeleton covered by thin skin, often spiny.

• Water Vascular System: Network of hydraulic canals powering tube feet for movement, feeding, and gas exchange.

• Tube Feet: Function like suction cups; used for locomotion and manipulating food (e.g., opening bivalve shells).