Lecture 6. Annelida

Phylum Annelida Overview


  • Course: Invertebrate ZoologyLecture by: K.Y. Ahmadova

Coelomata

  • Group: Protostomia

    • Mouth forms at the site of blastopore

    • Mesoderm forms in teloblastic way

  • External skeleton: single-layer covering

  • Major phylums:

    • Annelids (Annelida)

    • Molluscs (Mollusca)

    • Arthropods (Arthropoda)

    • Onychophora (Onychophora)

General Features of Phylum Annelida

  • Body Structure:

    • Head lobe (prostomium), segmented trunk, anal lobe (pygidium)

  • Metamerism: Visible in both external and internal structure

  • Body cavity: secondary, lobes lack coelom

  • Musculocutaneous sac development:

    • Composed of epithelium, annular, and longitudinal muscles

  • Digestive System:

    • Divided into three acts with developed salivary glands

  • Excretory System: Nephridial type

  • Circulatory System: Typically closed, may be absent in some species

  • Respiratory System: May have gills, or respiration occurs through body surface

  • Nervous System: Consists of paired brain with abdominal nerve cord or ladder type

  • Reproductive Modes: Diecious or hermaphrodite, including spiral egg crushing, development with or without metamorphosis

Classification of Annelids

Class Archiannelida

  • Marine species, sizes up to 2-3 mm

Class Polychaeta (Bristle worms)

  • Mostly marine and free-living, some are parasitic

  • Subclasses:

    • Errantia: Stray forms

    • Sedentaria: Sedentary forms

Class Oligochaeta

  • Inhabit freshwater and soil, some are marine

  • Includes Orders Naidomorpha and Lumbricomorpha

Class Hirudinea (Leeches)

  • Freshwater or marine; ectoparasites, predators

  • Subclasses:

    • Archihirudinae: Ancient Leech

    • Euchirudinae: True leeches

External Structure of Polychaeta

  • Body parts: head, trunk, and anal lobe

  • Head includes eyes, antennae (tentacles), and olfactory pits

  • Trunk is segmented (homonomous/heteronomous) with parapodia (primitive limbs)

Internal Structure of Annelids

  • Muscular system includes circular and longitudinal muscles

  • Body cavity: Secondary coelom with specific layers (somatopleura and splanchnopleura)

Digestive System

  • Composed of fore, middle, and hind intestines

  • Cavity digestion with specialized structures for nutrient absorption

Excretory System

  • Nephridial in structure with variations across classes

Circulatory System

  • Closed system with dorsal and abdominal vessels and capillary networks

Nervous System

  • Central system includes paired cerebral ganglia and abdominal nerve cord

  • Evolution shows transition from orthogon to chain systems

Reproductive System

Polychaetes

  • Dioecious with external fertilization

Oligochaetes

  • Hermaphrodite with a complex reproductive structure including male and female organs

Leeches

  • Similar to oligochaetes, hermaphrodite with internal fertilization

Significance of Annelids

  • Provide food for various animals and humans

  • Aid in purification of water and soil formation

  • Earthworms used in agriculture and earth reclamation efforts.