Phylum Annelida

Phylum Annelida

  • General Characteristics

    • Segmented worms with a basic plan that features a segmented body.

    • Both internal and external structures are repeated in each segment, concept known as serial homology.

    • In annelids, serial homology leads to Metamerism, aiding in flexibility and independent segment functioning.

Taxonomic Groups

  • Classes of Annelids:

    • Class Polychaeta: Diverse group with numerous orders.

    • Class Clitellata: earthworms, leeches, related forms

    • Subclass Oligochaeta: Includes earthworms and others.

    • Subclass Hirudinoidea: Includes leeches.

Morphological Features

Class Polychaeta

  • Body Structure:

    • Head region has specialized features: prostomium (with tentacles), eyes, and palps.

    • Each body segment bears a pair of parapodia, which aid in movement and respiration.

Class Clitellata

  • Body Structure:

    • Segments modified into a clitellum, which is a secretory region important in reproduction.

    • Subclass Oligochaeta: Simple body, typical features include mouth, anus, and female/male pores.

    • Subclass Hirudinoidea: Fixed number of segments, usually 34, with flattened body and clitellum only visible during reproduction.

Locomotion and Support

  • Class Polychaeta:

    • Utilize coelomic spaces as a hydrostatic skeleton; each coelomic space can be hydraulically isolated for mobility.

    • Muscle contractions (longitudinal and parapodial) play a vital role in locomotion.

    • Methods of movement include crawling and swimming, adapted for burrowing.

  • Class Clitellata:

    • Rely on peristalsis and chaetal manipulation for movement; lack parapodial paddles.

    • Hydrostatic skeleton utilized for support, dependent on transverse and longitudinal muscle contractions.

Feeding Mechanisms

  • Class Polychaeta:

    • Diverse feeding strategies: raptorial, deposit, and suspension feeding.

    • Many are predators with specialized feeding organs like evertable pharynx and jaws.

  • Subclass Oligochaeta:

    • Primarily detritivorous or deposit feeders with less diverse feeding habits than polychaetes.

  • Class Hirudinoidea:

    • Ectoparasitic or predators, with some members utilizing a proboscis or jaws to feed on blood or small invertebrates.

    • Produce Hirudin, an anticoagulant, for feeding on blood.

Circulation and Gas Exchange

  • Closed Circulatory System:

    • Annelids possess a closed circulatory system comprised of vessels running along the body and into appendages.

    • Oxygenation occurs usually at the body surface, via gills or skin.

Excretion and Osmoregulation

  • Metanephridia in each segment help expel metabolic waste and regulate osmotic balance within coelomic cavity.

Nervous System and Sense Organs

  • Annelids have a generalized nervous system structure comprising dorsal cerebral ganglion, paired connectives, and ventral nerve cords with segmental ganglia.

Reproduction

  • Polychaetes:

    • Dioecious, lacking permanent gonads, with gametes forming in the coelom.

    • Fertilization can be either internal or external.

    • Development includes free-swimming trochophore larva.

  • Oligochaeta:

    • Hermaphroditic with complex reproductive systems.

    • Require physical contact for fertilization; reproduction occurs under specific environmental conditions.

    • Clitellum aids in sperm exchange and cocoon formation.

  • Hirudinoidea:

    • Exhibit similarities to oligochaetes in reproduction and development.

    • Details on regeneration and asexual reproduction are currently unclear.