An Introduction to Invertebrates: Diversity, Structure, and Evolution

Chapter 33: An Introduction to Invertebrates

Overview of Invertebrate Diversity

Invertebrates are animals that lack a backbone and represent over 95% of all known animal species. They inhabit nearly every environment on Earth and display remarkable morphological diversity, ranging from microscopic forms to species up to 18 meters long. Understanding invertebrate diversity involves classifying them into groups based on evolutionary relationships, which helps clarify their vast variety of forms and functions.

• Kingdom Animalia contains approximately 1.3 million described species, with estimates of total species ranging up to 10–20 million.

• Major invertebrate groups include Porifera (sponges), Cnidaria (jellies, corals), Lophotrochozoa (flatworms, molluscs, annelids), Ecdysozoa (arthropods, nematodes), and Deuterostomia (echinoderms, some chordates).

Main Invertebrate Phyla and Their Characteristics

Invertebrates are classified into several major phyla, each with unique structural and functional features. The following table summarizes key phyla and their representative species:

Phylum	Species Estimate	Key Features	Example
Porifera	5,500	No true tissues, filter feeders	Sponge
Cnidaria	10,000	Radial symmetry, diploblastic, gastrovascular cavity	Jellyfish
Acoela	400	Simple nervous system, saclike gut	Acoel flatworm
Placozoa	1	Bilayer of cells, asexual reproduction	Trichoplax adhaerens
Ctenophora	100	Diploblastic, radial symmetry, ciliated combs	Comb jelly

Porifera (Sponges)

Sponges are among the simplest animals, lacking true tissues and organs. They are mostly marine and are sedentary filter feeders.

• Body Structure: Sac-like body with pores, central cavity (spongocoel), and an opening (osculum).

• Cell Types: Choanocytes (collar cells) for feeding, amoebocytes for digestion and support.

• Reproduction: Most are hermaphrodites, with sequential hermaphroditism common.

Cnidaria

Cnidarians include corals, hydras, and jellies. They are diploblastic and radially symmetrical, with a central gastrovascular cavity.

• Body Forms: Sessile polyp and motile medusa.

• Feeding: Tentacles with cnidocytes (stinging cells) and nematocysts for prey capture.

• Nervous System: Noncentralized nerve net and sensory structures.

Other Basal Animal Phyla

• Acoela: Simple flatworms with a saclike gut and basic nervous system.

• Placozoa: Only one known species, Trichoplax adhaerens, with a simple bilayer structure.

• Ctenophora: Comb jellies with eight rows of cilia for movement; important planktonic animals.

Lophotrochozoa

Lophotrochozoa is a diverse clade identified by molecular data, including flatworms, rotifers, ectoprocts, brachiopods, molluscs, and annelids. Members may have a lophophore (feeding structure) or trochophore larval stage.

• Platyhelminthes: Flatworms with bilateral symmetry, no body cavity, and a branched gastrovascular cavity.

• Syndermata: Includes rotifers (microscopic, complex organ systems) and acanthocephalans (parasitic).

• Ectoprocta: Sessile, colonial animals with a tough exoskeleton.

• Brachiopoda: Marine animals with dorsal and ventral shells, lophophore for feeding.

• Gastrotricha: Tiny, ciliated worms living at the bottom of aquatic habitats.

• Cycliophora: Symbionts on lobsters, unique reproductive strategies.

• Nemertea: Ribbon worms with a proboscis for prey capture and a closed circulatory system.

• Annelida: Segmented worms, including earthworms and leeches.

• Mollusca: Soft-bodied animals, often with a hard shell; includes snails, clams, squids, and octopuses.

Ecdysozoa

Ecdysozoans are characterized by a tough external cuticle that is periodically shed (ecdysis). This group includes more species than all other eukaryotic groups combined.

• Loricifera: Tiny marine animals with a protective lorica.

• Priapula: Marine worms with a large proboscis.

• Tardigrada: Water bears, known for their resilience to extreme conditions.

• Onychophora: Velvet worms, originally marine, now terrestrial.

• Nematoda: Roundworms, abundant in soil and aquatic habitats, many are parasitic.

• Arthropoda: The largest animal phylum, including insects, crustaceans, and arachnids, all with segmented exoskeletons and jointed appendages.

Summary Table: Major Invertebrate Groups

Group	Key Features	Representative Species
Porifera	No true tissues, filter feeders	Sponge
Cnidaria	Radial symmetry, stinging cells	Jellyfish
Lophotrochozoa	Diverse body plans, lophophore/trochophore	Flatworm, Rotifer, Mollusc, Annelid
Ecdysozoa	Molting cuticle, jointed appendages	Roundworm, Arthropod
Deuterostomia	Includes invertebrates and vertebrates	Echinoderm, Chordate

Key Terms and Concepts

• Invertebrate: Animal lacking a backbone.

• Filter feeder: Organism that obtains food by filtering water for particles.

• Diploblastic: Having two germ layers (ectoderm and endoderm).

• Bilateral symmetry: Body plan with right and left halves that are mirror images.

• Lophophore: Crown of ciliated tentacles for feeding.

• Ecdysis: Shedding of the cuticle during growth.

Example: The phylum Arthropoda includes insects, which are the most diverse group of animals on Earth, occupying nearly every habitat and playing crucial ecological roles as pollinators, decomposers, and food sources for other animals.

Additional info: This summary covers the main invertebrate phyla, their evolutionary relationships, and key structural and functional adaptations, providing a foundation for further study of animal diversity and evolution.