Chapter 33: An Introduction to Invertebrates - Study Guide

Chapter 33: An Introduction to Invertebrates

Sponges: Basal Animals

Sponges are considered basal animals, representing the earliest and simplest form of animal life. They lack true tissues and organs, distinguishing them from other animal groups.

• Basal animals: Animals that diverged early in animal evolution.

• Absence of tissues: Sponges do not possess organized tissues.

• Filter feeders: Sponges filter water to obtain food particles.

Phylum Porifera

Sponges belong to the phylum Porifera. Members of this phylum are characterized by their porous bodies and simple structure.

• Porous body: Numerous pores allow water to circulate through the sponge.

• Choanocytes: Specialized cells that generate water flow and capture food.

• Asymmetrical: Sponges lack symmetry.

Phylum Cnidaria

Cnidarians are a diverse group of aquatic animals known for their radial symmetry and specialized stinging cells.

• Radial symmetry: Body parts arranged around a central axis.

• Cnidocytes: Cells containing nematocysts (stinging organelles).

• Diploblastic: Two tissue layers: ectoderm and endoderm.

• Examples: Jellyfish, sea anemones, corals.

Phylum with Widest Range of Animal Body Forms: Phylum Mollusca

Mollusca is notable for its diversity in body forms, ranging from snails to octopuses.

• Triploblastic: Three tissue layers: ectoderm, mesoderm, endoderm.

• Coelomate: Possess a true body cavity (coelom).

• Special structures: Some have a radula (feeding organ), others have a trochophore (larval stage), and some have neither.

• Characteristics: Soft-bodied, often protected by a hard shell.

Lophophorates

Lophophorates are a subgroup within certain animal phyla, characterized by a lophophore, a specialized feeding structure.

• Members: Brachiopoda and Ectoprocta (Bryozoans).

• Brachiopoda: Resemble clams, have two shells, use lophophore for feeding.

• Ectoprocta: Colonial, encased in exoskeleton, use lophophore for feeding.

Phylum Mollusca

Mollusca includes a wide variety of organisms, most of which are protected by a hard shell.

• Examples: Snails, clams, octopuses, squids.

• Protection: Most have a calcium carbonate shell.

• Three main structures:

  • Muscular foot: Used for movement.

  • Visceral mass: Contains internal organs.

  • Mantle: Secretes the shell.

Major Classes of Mollusca

Mollusca is divided into four major classes, each with unique features.

• Polyplacophora: Chitons; have eight plates, marine.

• Gastropoda: Snails and slugs; single, spiraled shell (or none), terrestrial and aquatic.

• Bivalvia: Clams, oysters, mussels; two shells, filter feeders.

• Cephalopoda: Octopuses, squids, cuttlefish; highly developed nervous system, tentacles, reduced or absent shell.

Phylum Annelida

Annelids are segmented worms with a true coelom.

• Segmentation: Body divided into repeated segments.

• Coelomate: True body cavity.

• Examples: Earthworms, leeches, polychaetes.

• Errantia: Mobile marine predators

• Parapodia: Pair of paddle-like structures

• Sedentarians: Burrow into the substrate

Phylum with Most Species-Rich Animal Group: Ecdysozoa

Ecdysozoa includes the most species-rich animal groups, characterized by molting their cuticle.

• Cuticle: Tough external covering.

• Molting (Ecdysis): Shedding of the cuticle.

• Responsible for: Nematoda and Arthropoda.

Phylum Nematoda

Nematodes, or roundworms, are unsegmented worms with a cylindrical body.

• Unsegmented: Smooth, cylindrical body.

• Pseudocoelomate: Body cavity not fully lined by mesoderm.

• Complete digestive tract: Mouth and anus.

• Examples: Caenorhabditis elegans, parasitic species.

Phylum Arthropoda

Arthropods are the most diverse animal phylum, with adaptations that allow them to occupy nearly all habitats.

• Cambrian explosion: Rapid diversification of arthropods.

• Habitat diversity: Found in terrestrial, aquatic, and aerial environments.

• Segmented body: Division into head, thorax, abdomen.

• Hard exoskeleton: Made of chitin, provides protection and support.

• Jointed appendages: Specialized for various functions.

Benefits of Exoskeleton

The exoskeleton provides several advantages to arthropods.

• Protection: Shields against predators and environmental hazards.

• Support: Facilitates movement and muscle attachment.

• Prevents desiccation: Reduces water loss.

Modern Lineages of Arthropods

Arthropods are divided into three main lineages, each with distinct characteristics.

Lineage	Examples	Highlighted Characteristics
Chelicerates	Spiders, scorpions, horseshoe crabs	Chelicerae (feeding appendages), no antennae
Myriapods	Centipedes, millipedes	Many body segments, one or two pairs of legs per segment
Pancrustaceans	Crustaceans (crabs, lobsters), insects	Two pairs of antennae (crustaceans), wings (insects)

Metamorphosis: Complete vs. Incomplete

Arthropods, especially insects, undergo metamorphosis during development.

• Complete metamorphosis: Four distinct stages: egg, larva, pupa, adult. Example: butterflies.

• Incomplete metamorphosis: Three stages: egg, nymph, adult. Nymph resembles adult. Example: grasshoppers.

Echinoderms and Chordates

Echinoderms and chordates are members of the phylum Deuterostomia, sharing developmental characteristics.

• Echinoderms: Marine animals with radial symmetry as adults, water vascular system. Examples: sea stars, sea urchins.

• Chordates: Possess a notochord, dorsal nerve cord, pharyngeal slits, and post-anal tail at some stage. Examples: vertebrates (fish, mammals).