Phylum Annelida

• Annelids have a complete digestive system with an oral cavity that leads to a pharynx.
• One of the defining characteristics of Phylum Annelida is the presence of one or more pairs of chitinous setae or chaetae.
• Setae— structure that project outward from the epidermis to provide traction.
• Setae is made up of beta-chitin (which is flexible and tough), strengthened with sclerotized protein or inorganic material (e.g. calcium carbonate).
• Setae's function is to form temporary attachment sites and prevent backsliding during locomotion in or within the substrate or burrow
• Now includes pogonophorans (now siboglinids), echiurans, and sipunculans which were all formerly given status as separate phyla
• Body segments annulated or ring like
• Segments and organ systems are serially repeating (metameric segmentation)
• Body consists of three regions, Anterior segment (prostomium), Posterior segment (pygidium), Metameric body segments in between
• burrowing activities constantly rework the substrate in which they thrive
• they may ingest and excrete large quantities of sediments or soils
• keep soils aerated, and their castings fertilize the soil
• Body wall is flexible, has important role on locomotion, thinner portions can serve as surface for gas exchange (moisture is a requisite), epidermal cells secrete cuticle, but remains permeable to both water and gas
• Collagenous cuticle

  ▪ Non-living layer

  ▪ Permeable

  ▪ Mainly for protection
• Simple Glandular Epidermis

  Living layer

  Layer where the chaetae is formed and anchored

  Chaetae forms from an invagination of the epidermal chaetoblast

  (Fauchald, 2013)

  Contains cells that secrete the cuticle

  Other cells: supporting cells, albumin cells, mucus cells, basal cells