Single body opening (e.g. hydra)

• what is hydra related to?

  its related to sea anemones and, like them, is diploblastic (i.e. it comprises of 2 layers of cells, an ectoderm and an endoderm, separated by a jelly layer containing a network of nerve fibres)
• what does the hydra comprise of?

  it comprises of 2 cells, an ectoderm and an endoderm, separated by a jelly layer containing a network of nerve fibres
• whats the structure of hydra?

  its cylindrical and has tentacles at the top (usually 6) surrounding its mouth, the only body opening
• where does hydra live?

  in fresh water, attached to leaves or twigs by a basal disc
• what happens when a hydra is hungry?

  • it extends its tentacles and when small organisms (e.g. daphnia, the water flea) brush against the tentacles, their stinging cells discharge and paralyse the prey
  • the tentacles move the prey through the mouth into the hollow body cavity
  • some endodermal cells secrete protease and lipase, though not amylase; the prey is digested extracellularly and the products of digestion are absorbed into the cells
  • other endodermal cells are phagocytic and engulf food particles, which they digest in food vacuoles
  • indigestible remains are egested through the mouth
• what do the tentacles of all 3 species of hydra contain and do?

  • in all 3 british species of hydra, as in sea anemones, the tentacles contain photosynthesising protoctista
  • experiments using radioactive carbon in carbon dioxide show that they pass sugars to hydra
• what is this an image of?

  the structure of a hydra