BIO 4

• Phylum Annelida


  Annelids are segmented worms

  
• Characteristics of annelids


  • Segmented bodies externally and internally
  • About 15,000 species ranging in length from less than 1 mm to 3 m
  • Live in salt-water, most freshwater habitats, and damp soil
  • Have a true body cavity (a coelom) and are triploblastic
  • Annelid body parts are repeated in each segment, each with own organs (METAMERISM)
  • Have antagonistic muscle layers (circular and longitudinal)
  • Have points of muscle attachment so as to apply force
  • Have a coelom that forms a hydroskeleton for movement
  • Have a nervous system that coordinates waves of movement
  • Have an advanced blood system that includes blood pigments
  • Have an excretory system
• Annelid worms


  • Creeping and burrowing animals
  • Some aquatic annelids swim in pursuit of food
  • Traditionally divided into three classes: Oligochaeta, Polychaeta, and Hirudinea
• Class Oligochaeta


  Coelom of the earthworm is partitioned by septa, but the digestive tract, longitudinal blood vessels, and nerve cords penetrate the septa and run the animal's length
• Coordination of locomotion in the Earthworm

  1. Stimulation to head - medial giant fibre - longitudinal contract - setae on tail protrude (FAST WITHDRAWL)

  2. Stimulation of tail - two lateral giant fibres - longitudinals contract (PROTRUSION OF HEAD)

  
• Digestive system of earthworm

  • Pharynx
  • Esophagus
  • Crop
  • Gizzard
  • Intestine
• Earthworm circulatory system


  • Closed circulatory system carries blood with oxygen-carrying hemoglobin through dorsal and ventral vessels connected by segmental vessels
  • Dorsal vessel and five pairs of esophageal vessels act as muscular pumps to distribute blood
• Earthworm excretory system


  • In each segment is a pair of excretory tubes, metanephridia, that remove wastes from the blood and coelomic fluid
  • Wastes are discharged through exterior pores

  
• Earthworm nervous system


  • A brainlike pair of cerebral ganglia lie above and in front of the pharynx
  • A ring of nerves around the pharynx connects to a subpharyngeal ganglion
• Earthworm reproduction


  • Earthworms are cross-fertilizing hermaphrodites
  • Two earthworms exchange sperm and then separate
  • The received sperm are stored while a special organ, the clitellum, secretes a mucous cocoon
  • As the cocoon slides along the body, it picks up eggs and stored sperm and slides off the body into the soil
  • Some earthworms can also reproduce asexually by fragmentation followed by regeneration

  
• Class Polychaeta


  • Each segment has a pair of paddlelike or ridgelike parapodia ("almost feet") that function in locomotion
  • Each parapodium has several chitinous setae
  • In many polychaetes, the rich blood vessels in the parapodia function as gills
  • Most polychaetes are marine and crawl on or burrow in the seafloor, a few drift and swim in the plankton, others live in tubes they make by mixing mucus with sand and broken shells
  • Polychaetes include carnivores, scavengers, and planktivores
  • The brightly coloured fanworms trap plankton on feathery tentacles

  
• Class Hirudinea


  • The majority of leeches inhabit fresh water, but land leeches move through moist vegetation
  • Leeches range in size from about 1 to 30 cm
  • Many leeches feed on other invertebrates, but some blood-sucking parasites feed by attaching temporarily to other animals, including humans
  • Some parasitic species use bladelike jaws to slit the host's skin, while others secrete enzymes that digest a hole through the skin
  • The leech secretes hirudin, an anticoagulant, into the wound, allowing the leech to suck as much blood as it can hold
  • Until this century, leeches were frequently used by physicians for bloodletting
  • Leeches are still used for treating bruised tissues and for stimulating the circulation of blood to fingers or toes that have been sewn back to hands or feet after accidents

  
• Movement in Leeches


  • Leeches are structually more complex and more specialised than other annelids, but are mechanically simpler
  • They have no septa
  • They have 34 segments without setae
  • The coelom is continuous and so is a fluid filled bag
  • Most do a simple creeping action (looping)
• Piscicola looping movement(leeches)


  1. Attaches by post sucker, extends body (circular muscles), attaches anterior sucker, repositions posterior sucker, contracts (longitudinal muscles) shortens body pulling body towards anterior sucker

  2. Touches the side of itself before reattaching then loops out to repeat the cycle again
• Evolutionary significance of the coelom

  • The coelom provides a hydrostatic skeleton that allows new and diverse modes of locomotion
  • It also provides body space for storage and for complex organ development
  • The coelom cushions internal structures and separates the action of the body wall muscles from those of the internal organs, such as the digestive muscles
• Segmentation


  • Allows a high degree of specialisation of body regions
  • Groups of segments are modified for different functions
• There are three classes of annelids

  Oligochaeta, Polychaeta, and Hirudinea.
• Annelid body parts are repeated in each segment, each with own organs. This is known as ....

  METAMERISM
• Annelids have a true body cavity (a coelom) and are triploblastic.