16C (bio)

• Roundworms

  • Ascaris, a parasitic roundworm that can reach a foot long
• Worms exhibit bilateral symmetry, but beyond this, there are many differences in their body plans and features
• Free-living worms tend to be more complex than parasitic ones, especially internal parasites
• Worms

  • Have a mouth at one end and an anus at the other end
  • Some have segmented bodies
• Worm digestive system

  1. Food is broken down by digestive system

  2. Nutrients are absorbed into cells that need them (extracellular digestion)
• Tapeworms lack a digestive system and absorb already digested nutrients from the host's intestines through their outer layer
• Worm nervous system

  • Consists of two bundles of nerve cells called ganglia
  • Outer layer has sensory organs to detect the environment
• Flatworms and roundworms lack a closed circulatory system, instead using diffusion to spread nutrients
• Most worms lack specialized respiratory organs, though some aquatic worms have gills
• Worms have structures to prevent osmosis and remove nitrogenous wastes
• Scolex


  The head of a tapeworm, a type of flatworm
• Tapeworm reproductive system

  • Has hooks and suckers to attach to host
  • Contains testes, uterus, and genital pore
• Most worms reproduce sexually, though some flatworms also reproduce asexually
• Hermaphroditic worms typically cross-fertilize with other individuals rather than self-fertilize
• Worm reproduction

  1. Lay eggs in cocoons or release plankton-like larvae

  2. Parasitic worms release eggs in host feces
• Planarian flatworms have remarkable powers of regeneration, able to regrow a whole new individual from a fragment
• Many worms are parasites, drawing nutrition from their hosts
• Parasitic worms have fewer sensory organs and reduced locomotion, but can avoid the host's immune system
• Parasitic worms often have complex life cycles involving multiple hosts
• The guinea worm is a roundworm that can infect a human, mating inside Its host's body and emerging, usually near the foot, in an extremely painful blister
• Though parasites can cause disease and are usually harmful, doctors are working on using parasitic worms to treat certain diseases
• Some experimental procedures deliberately infect ill people, suffering from problems caused by an overactive immune system, with certain types of parasitic worms
• The worms seem to decrease the body's immune response, giving relief to the patients
• Doctors have been using leeches for medical purposes for centuries
• Leech saliva keeps the blood from clotting, which allows the blood vessels of the two pieces of skin to grow together
• The leeches can be raised in sterile environments so that they do not introduce the risk of infection
• Earthworms eat decaying organic matter in the soil, taking in some soil along with the decayed material to help grind it up
• This not only sustains the worms but also enriches the soil as the earthworm expels its wastes, called castings
• Earthworm burrows also allow oxygen and water to filter down into the ground, enabling plants to send their roots down deeper
• A single earthworm can produce one half pound of rich, fertile soil in one year
• Tens of thousands of earthworms can inhabit one acre of land, having a significant impact on an area's ecology