Annelids

• Annelids are protostome coelomates in superphylum Lophotrochozoa.
• Annelids have spiral and determinate cleavage.
• Annelids are segmented worms.
• Annelids have bodies composed of a series of fused rings.
• The evolutionary innovation shown by annelids is segmentation (metamerism).
• The body of Annelids is divided into a series of segments, each having similar components of all major organ systems.
• Many annelids have chitinous bristles called setae.
• Annelids can be found worldwide in marine, freshwater, and terrestrial habitats.
• Prostomium – anterior part followed by segmented body.
• Pygidium – terminal portion.
• Peritonia (layers of mesodermal epithelium) of adjacent segments meet to form septa.
• Fluid-filled coelom acts as a hydrostatic skeleton.
• Polychaeta is the largest, most diverse class.
• Polychaeta may be brightly colored, variable shape.
• Polychaetes have a well developed head.
• Polychaetes have paired appendages, parapodia, that function as gills and aid in locomotion.
• Polychaetes have no clitellum
• Polychaetes are mostly marine and mostly benthic.
• Polychaetes may live under rocks, burrow into sediment, or build their own tubes
• Some polychaetes are planktonic
• Sedentary forms of Polychaetes are filter or deposit feeders
• Errant forms of Polychaetes include pelagic and benthic types and are often predators or scavengers.
• Gonads are temporary structures in polychaetes.
• Gonads of Polychaetes appear as simple temporary swellings of peritoneum
• In Polychaetes, sexes are usually separate and fertilization is external.
• Early larva of Polychaetes is a trochophore
• Many polychaetes have respiratory pigments - Hemoglobin, chlorocruorin or hemerythrin.
• Most polychaetes have parapodia and gills for gaseous exchange and others use the body surface.
• Excretory organs consist of protonephridia and mixed proto and metanephridia in some, but most polychaetes have metanephridia
• In some polychaetes, septa are incomplete and coelomic fluid serves circulatory function.
• Double ventral nerve cord runs length of the polychaete worm with ganglia in each metamere.
• In Polychaetes, selective resorption occurs along the nephridial duct.
• Sense organs of Polychaetes include: eyes, nuchal organs, and statocysts.
• Nuchal organs are ciliated sensory pits that are probably chemoreceptive.
• Some burrowing and tube-building polychaetes use statocysts to orient their body.
• Clam worms (Nereis) are errant polychaetes that live in mucus-lined burrows near low-tide level.
• Prostomium of Clam worms bears a pair of palps sensitive to touch and taste, a pair of short sensory tentacles, and two small dorsal eyes sensitive to light.
• Peristomium of Clam Worms has a ventral mouth, a pair of jaws, and four pairs of sensory tentacles.
• Scale worms are flattened bodies that are covered with broad scales
• Fireworms have hollow, brittle setae that contain poisonous secretions.