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Annelida

Chapter 11: Phylum Annelida

Overview

  • Phylum Name: Annelida (meaning "small rings")

  • Species Count: Approximately 6,000 species

  • Habitat: Aquatic and terrestrial

  • Diet: Scavengers, blood feeders, carnivores, filter feeders

  • Key Feature: Well-developed coelom (body cavity) and segmented bodies (metamerism)

Classification

  • Life Domain: Eukarya

  • Kingdom: Animalia

  • Phylum: Annelida

  • Classes within Phylum:

    • Errantia

    • Sedentaria

    • Clitellata

Developmental Biology

  • Metazoan Classification:

    • Protostomia vs Deuterostomia

    • Lophotrochozoa (including annelids and molluscs)

    • Ecdysozoa (including arthropods and nematodes)

  • Key Developmental Terms:

    • Multicellularity

    • Diploblasty

    • Triploblasty (formation of three germ layers—endo-, ecto-, and mesoderm)

    • Cephalization (development of a head)

    • Trochophore larva (larval stage in some annelids and molluscs)

Anatomy of Annelids

  • Segmentation (Metamerism):

    • Repetition of similar body segments along the longitudinal axis; segments are called metameres.

    • Initially identical in early development; gene expression leads to differentiation later.

Coelom Structure

  • Annelids possess a large coelom divided by septa, providing hydrostatic skeleton support.

  • Mesenteries support the digestive tract, while the peritoneum lines the coelomic cavity.

Muscular System

  • Movement Mechanism:

    • Use of circular and longitudinal muscles combined with setae to facilitate peristalsis and burrowing.

    • Alternating muscle contractions allow for movement and anchoring in the substrate.

Circulatory System

  • Type: Closed circulatory system

  • Components: Blood with hemoglobin for oxygen transport, leading to efficient nutrient delivery throughout the body.

Excretory System

  • Metanephridia:

    • Type of tubular nephridia present in each segment, responsible for excretion and osmoregulation.

    • Functions involve draining coelomic fluid and waste elimination.

Digestive System

  • Structure: Complete digestive tract (mouth to anus)

  • Extracellular digestion occurs throughout the digestive system, which includes structures like the esophagus, crop, gizzard, and intestine.

Nervous System

  • Cephalization: Formation of a brain and ventral nerve cord.

Reproductive System

  • Classes and Notable Features:

    • Class Errantia: Mostly marine, mobile predatory worms with well-defined morphology and reproductive behaviors.

    • Class Sedentaria: Mostly sessile, featuring varied feeding habits; includes leeches and earthworms.

    • Order Clitellata: Notable features are the presence of the clitellum involved in reproduction, with some being primarily terrestrial (earthworms).

Ecological Role

  • Importance of Annelids:

    • Benthic predators and contributors to soil health; enhance soil porosity and water capacity

    • Play critical roles in detritus processing, aiding decomposition and recycling in ecosystems.

    • However, many North American species are invasive, altering native landscapes.