Animal Diversity: Ecdysozoa, Deuterostomes, and Chordates

Animal Diversity: Ecdysozoa, Deuterostomes, and Chordates

Introduction

This section explores the major animal phyla, focusing on the Ecdysozoa (including nematodes and arthropods), Deuterostomes (including echinoderms and chordates), and the evolutionary relationships among these groups. Key developmental and anatomical features are highlighted, with emphasis on their relevance to microbiology, genetics, and evolutionary biology.

Ecdysozoa

Overview of Ecdysozoa

• Ecdysozoa is a major clade of protostome animals characterized by periodic molting of their exoskeleton (a process called ecdysis).

• Includes phyla such as Nematoda (roundworms) and Arthropoda (insects, spiders, crustaceans, etc.).

Nematodes (Roundworms)

• Bilateral, triploblastic, protostome animals.

• Possess a tough, non-living cuticle (exoskeleton) that must be shed for growth (ecdysis).

• Exoskeleton is abiotic and completely encases the animal, preventing mollusc-type growth.

• Cannot grow like annelid worms; must molt to increase in size.

Example: Caenorhabditis elegans is a non-parasitic nematode, widely used as a genetic model organism. Every cell in its body is mapped, and it has a sequenced, editable genome.

• Hermaphrodites: 959 total cells, 302 nerve cells

• Males: 1031 total cells, 385 nerve cells

Arthropods

• Bilateral, triploblastic, protostome animals.

• Possess a molting exoskeleton and often undergo metamorphosis.

• Segmented body plan; segments are more specialized than in annelids.

• Segments fuse to form major body regions (e.g., head, thorax, abdomen).

• Jointed appendages (legs, antennae).

• Open circulatory system.

Main Groups of Arthropods

• Chelicerates: Characterized by chelicerae (pincer or fang-like appendages), two primary body parts (cephalothorax and abdomen). Includes arachnids (spiders, scorpions, ticks, mites) and horseshoe crabs.

• Myriapods: Less specialized segments. Includes millipedes (herbivores, two pairs of legs per segment) and centipedes (carnivores, one pair of legs per segment, often venomous).

• Pancrustaceans: Includes crustaceans and insects, now recognized as a single clade. Insects are the most diverse group.

Insect Orders

• Insects have either zero or two pairs of wings, but some have modified wings.

• Beetles: Front wings are hard plates (elytra) protecting hindwings.

• Flies: Hindwings reduced to stubs (halteres) for balance.

• Major orders are classified by metamorphosis type (hemimetabolous: incomplete; holometabolous: complete) and wing presence.

Deuterostomes

Overview of Deuterostomes

• Includes Echinoderms and Chordates.

• Characterized by deuterostome development: the blastopore becomes the anus during gastrulation.

• Triploblastic and bilaterally symmetrical (at least in larvae).

Echinoderms

• Larvae are bilaterally symmetrical; adults are mostly radially symmetrical (a novel trait).

• Possess an endoskeleton with extensions forming external spines.

• Most move using tube feet and a water vascular system.

Examples:

• Sea stars: Crawl along rock and coral, hunt clams.

• Sand dollars: Bury in sand, eat small crustaceans, plankton, algae, detritus.

• Sea urchins: Eat algae, can be painful to step on.

• Sea lilies: Suspension feeders.

• Sea cucumbers: Eat plankton and detritus, some are eaten by humans.

• Brittle stars: Locomote by thrashing arms, some are bioluminescent.

Chordates

Defining Characteristics

• Bilateral, triploblastic, deuterostome animals.

• All chordates share:

  • Dorsal, hollow nerve cord

  • Notochord (flexible rod for support)

  • Pharyngeal slits (may become filter-feeding organs or respiratory organs)

  • Muscular post-anal tail

Chordate Body Plan

• Dorsal, hollow nerve cord (unique to chordates; other phyla have ventral, solid nerve cords).

• Notochord provides support and muscle attachment.

• Pharyngeal slits and post-anal tail are present at some stage in development.

Groups of Chordates

• Invertebrate chordates:

  • Lancelets: Live in marine sand, suspension feeders, adults retain chordate features.

  • Tunicates (sea squirts): Sessile suspension feeders, larvae have chordate features, adults are mostly bilaterally symmetric.

• Vertebrate chordates: Distinguished by the presence of a vertebral column, neural crest cells, and an extensive set of Hox genes.

Comparative Table of Major Animal Phyla

Phylum	Symmetry	Tissues	Body Cavity	Development	Digestive Openings	Examples
Sponges	None	No	None	None	Many pores	Sponges
Cnidarians	Radial	Yes	None	None	One	Jellyfish, corals
Flatworms	Bilateral	Yes	Acoelomate	Protostome	One	Planaria
Molluscs	Bilateral	Yes	Coelomate	Protostome	Two	Snails, clams
Annelids	Bilateral	Yes	Coelomate	Protostome	Two	Earthworms
Nematodes	Bilateral	Yes	Pseudocoelomate	Protostome	Two	Roundworms
Arthropods	Bilateral	Yes	Coelomate	Protostome	Two	Insects, spiders
Echinoderms	Radial (adult), Bilateral (larva)	Yes	Coelomate	Deuterostome	Two	Sea stars, sea urchins
Chordates	Bilateral	Yes	Coelomate	Deuterostome	Two	Vertebrates, lancelets

Key Developmental Terms

• Ectoderm: Outer germ layer; forms skin and nervous tissue.

• Endoderm: Inner germ layer; forms the digestive tract.

• Mesoderm: Middle germ layer; forms muscles and other organs.

• Protostome: The blastopore becomes the mouth during development.

• Deuterostome: The blastopore becomes the anus during development.

Sample iClicker Questions (with Answers)

• Typical animal embryos have an ectoderm (which makes skin and nervous tissue) and an endoderm (which makes the digestive tract).

• Coral reefs are hard external skeletons secreted by certain cnidarians.

• In protostomes, the opening that forms during gastrulation becomes the mouth.

Additional info: This summary expands on the provided slides by clarifying developmental biology terms, adding context for model organisms, and providing a comparative table for major animal phyla.