Water excretion by hydra.

@article{Macklin1973WaterEB,
  title={Water excretion by hydra.},
  author={Martin Macklin and T Roma and Kendra Drake},
  journal={Science},
  year={1973},
  volume={179 4069},
  pages={194-5}
}
Hydra were cut so that regenerates consisting only of the central gastric region were formed. This region, which has no natural opening to the environment, is capable of osmoregulation and of removing excess fluid from the gut. The fluid is excreted through a break in the body wall created as a result of a strong contraction when the gut is distended with fluid. A normal hydra, therefore, must remove excess fluid by contracting and expelling it through its mouth.