Phylum Annelida and Its Classes

Phylum Annelida

• Annelids are a phylum of segmented worms
• Number of described species is approximately 12,400
• Defining characteristic: One or more pairs of chitinous setae (chaetae)
• Body structure: Soft-bodied, reasonably circular in cross-section, and longer than wide
• Metamerism: Series of repeating segments with repeating organ systems (skin, musculature, nervous system, and excretory system)

Class Polychaeta

• Defining characteristic: Paired lateral outfoldings of the body wall (parapodia); many setae
• Distribution: Primarily marine (63% of annelid species)
• Body features: Parapodia function in gas exchange and locomotion; setae are siliceous, chitinous, or calcareous bristles
• Feeding: Predation, scavenging, suspension feeding, and deposit feeding
• Reproduction/development: Includes active, mobile species (errant) and sedentary species that live within protective tubes

Class Oligochaeta

• Defining characteristics: Pronounced cylindrical glandular region of the body (clitellum) that plays important roles in reproduction and permanent gonads
• Distribution: Primarily freshwater or terrestrial habitats (6.5% of annelid species, with few marine)
• Body features: Lack parapodia, protosomium lacks conspicuous sensory structures, setae are less densely distributed, and gas exchange occurs through diffusion across a moist body wall
• Movement: Continuous cycles of contraction and relaxation of circular and longitudinal muscles called peristaltic waves
• Reproduction: Hermaphrodites, sperm simultaneously exchanged, sperm stored in specialized organs called spermathecae, fertilization occurs externally and embryos develop in a nutritive fluid within a cocoon. Asexual reproduction is more common, involves transverse division of the body into segments and subsequent regeneration

Class Hirudinea

• Defining characteristic: Posterior sucker
• Distribution: Mostly freshwater and terrestrial habitats with fewer marine species
• Body features: Lack parapodia, specialized respiratory structures, and head appendages; the body is often not separated into segments by septa with a continuous coelomic space largely filled with connective tissue (mesenchyme), and the circulatory medium is the coelomic fluid
• Locomotion: Move by generating peristaltic waves, using suckers as temporary anchors
• Feeding: Ectoparasitic, feeding on the blood of other invertebrates and vertebrates, possess 3 toothed jaws for making an incision on the surface of the host.
• Reproduction: Simultaneous hermaphrodites with internal fertilization, the clitellum functions in producing a cocoon and a nutritive fluid, miniature leech emerges from the cocoon after development