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.