An Introduction to Invertebrates

An Introduction to Invertebrates

This chapter provides an overview of invertebrate diversity, their evolutionary relationships, and the major animal phyla that lack backbones. Invertebrates represent the vast majority of animal species and display a remarkable range of body forms, life cycles, and adaptations.

Key Concepts

• 33.1 Sponges are basal animals that lack tissues.

• 33.2 Cnidarians are an ancient phylum of eumetazoans.

• 33.3 Lophotrochozoans are the most diverse bilaterian clade, identified by molecular data.

• 33.4 Ecdysozoans are the most species-rich animal group.

• 33.5 Echinoderms and chordates are deuterostomes.

Exploring Invertebrate Diversity

Invertebrates are animals without a backbone, comprising over 95% of all known animal species. They are found in nearly every habitat on Earth and display a wide variety of body plans and adaptations.

Major Invertebrate Phyla

Phylum	Approximate Number of Species	Key Features
Porifera	5,500	Sponges; lack true tissues; filter feeders
Placozoa	1	Simple, flat animals; few cell types
Cnidaria	10,000	Jellyfish, corals; stinging cells (cnidocytes); radial symmetry
Ctenophora	100	Comb jellies; cilia for movement
Acoela	400	Simple, flatworms; lack true gut cavity
Platyhelminthes	20,000	Flatworms; bilateral symmetry; some parasitic
Syndermata	2,900	Rotifers and acanthocephalans; pseudocoelomates
Ectoprocta	4,500	Colonial, filter-feeding animals
Brachiopoda	335	Lamp shells; resemble clams
Gastrotricha	800	Tiny, aquatic animals; cilia-covered
Cycliophora	1	Live on lobster mouthparts
Nemertea	900	Ribbon worms; proboscis for feeding
Annelida	16,500	Segmented worms; earthworms, leeches
Mollusca	100,000	Snails, clams, squids; soft-bodied, often with shells
Loricifera	10	Tiny, marine sediment dwellers
Priapula	16	Penis worms; marine, burrowing
Onychophora	110	Velvet worms; segmented, soft-bodied
Tardigrada	800	Water bears; extremophiles
Nematoda	25,000	Roundworms; many parasitic
Arthropoda	1,000,000+	Insects, spiders, crustaceans; exoskeleton, jointed appendages
Hemichordata	85	Acorn worms; marine, burrowing
Chordata	57,000	Includes all vertebrates and some invertebrates
Echinodermata	7,000	Sea stars, sea urchins; radial symmetry as adults

Concept 33.1: Sponges are Basal Animals That Lack Tissues

Sponges (phylum Porifera) are simple, multicellular animals that lack true tissues and organs. They are sessile filter feeders, drawing water through their porous bodies to capture food particles.

• Structure: Body consists of two cell layers separated by a gelatinous matrix; key cell types include choanocytes (collar cells) and amoebocytes.

• Feeding: Water flows in through pores, food particles are trapped by choanocytes, and waste exits through the osculum.

• Reproduction: Can reproduce both sexually and asexually.

Example: Bath sponges used by humans are skeletons of marine sponges.

Concept 33.2: Cnidarians are an Ancient Phylum of Eumetazoans

Cnidarians (phylum Cnidaria) include jellyfish, corals, and hydras. They are radially symmetrical and possess true tissues, including a simple nerve net.

• Body Forms: Two main forms: polyp (sessile) and medusa (motile).

• Key Feature: Cnidocytes—specialized stinging cells containing nematocysts for prey capture and defense.

• Life Cycle: Many alternate between polyp and medusa stages (e.g., Obelia).

Example: Coral reefs are built by colonial cnidarians that secrete calcium carbonate skeletons.

Concept 33.3: Lophotrochozoans—A Clade Identified by Molecular Data

Lophotrochozoa is a diverse group of bilaterian animals, including flatworms, rotifers, molluscs, and annelids. They are united by molecular evidence and some developmental features (lophophore feeding structure or trochophore larva).

Flatworms (Platyhelminthes)

• Body Plan: Bilateral symmetry, acoelomate, dorsoventrally flattened.

• Types: Free-living (planarians) and parasitic (tapeworms, flukes).

• Example: Planarians are model organisms for regeneration studies.

Rotifers and Acanthocephalans

• Rotifers: Tiny pseudocoelomates with a ciliated corona for feeding.

• Acanthocephalans: Parasitic, spiny-headed worms.

Lophophorates: Ectoprocts and Brachiopods

• Ectoprocts: Colonial, filter-feeding animals with a lophophore.

• Brachiopods: Marine animals with two shells, superficially resembling clams.

Molluscs

• Body Plan: Muscular foot, visceral mass, mantle (may secrete shell).

• Major Groups: Gastropods (snails, slugs), bivalves (clams, oysters), cephalopods (squids, octopuses).

• Adaptations: Radula for feeding, closed circulatory system in cephalopods.

Annelids

• Body Plan: Segmented worms; includes earthworms, leeches, and polychaetes.

• Earthworms: Important for soil aeration and nutrient cycling.

Concept 33.4: Ecdysozoans—The Most Species-Rich Animal Group

Ecdysozoa includes animals that grow by molting their exoskeleton (ecdysis). The two largest phyla are nematodes and arthropods.

Nematodes (Roundworms)

• Body Plan: Unsegmented, pseudocoelomate, covered by a tough cuticle.

• Ecology: Free-living and parasitic species; important in soil and as parasites of plants and animals.

Arthropods

• Key Features: Segmented body, jointed appendages, exoskeleton made of chitin.

• Major Groups: Chelicerates (spiders, scorpions), myriapods (centipedes, millipedes), crustaceans (crabs, lobsters), insects.

• Success: Arthropods are the most diverse and abundant animal group, occupying nearly every habitat.

Maximizing Surface Area

Many invertebrates maximize surface area for exchange of gases, nutrients, and wastes. Strategies include flattening, folding, branching, and projections.

• Equation for Surface Area of a Cube:

$SA = 6s^2$

• Equation for Volume of a Cube:

$V = s^3$

As organisms increase in size, their volume grows faster than their surface area, influencing the evolution of body plans and organ systems.

Summary Table: Major Invertebrate Groups

Group	Key Features	Examples
Porifera	No true tissues, filter feeders	Sponges
Cnidaria	Radial symmetry, stinging cells	Jellyfish, corals
Platyhelminthes	Flatworms, acoelomate	Planarians, tapeworms
Mollusca	Soft body, often with shell	Snails, clams, squids
Annelida	Segmented worms	Earthworms, leeches
Nematoda	Roundworms, cuticle	Ascaris, C. elegans
Arthropoda	Exoskeleton, jointed legs	Insects, spiders, crabs

Additional Info

• Invertebrates are critical to ecosystems as decomposers, pollinators, and food sources for other animals.

• Many invertebrates have complex life cycles with larval and adult stages adapted to different environments.