Hydra
- Overview of this freshwater polyp with photographs of a budding specimen and several closeup views.
- http://www.microscopy-uk.org.uk/mag/articles/hydra.html

Hydras
- Photographic study by Jan Parmentier of these organisms, with images of various reproductive structures.
- http://www.microscopy-uk.org.uk/mag/artjun98/janhydra.html

Hydra Oligactis
- Rosemarie Arbur has made a series of drawings of a hydra devouring a water flea.
- http://www.microscopy-uk.org.uk/mag/artsep01/rahydra.html

Image gallery: Hydra
- Includes downloadable video clips of the behavior of a typical hydra extending and contracting.
- http://www.microscopy-uk.org.uk/mag/artoct03/dfhydra.html

The Hydrozoa Directory
- Information about the classification, anatomy, and life cycle of this class, including photographs and diagrams. Also provides biographies and photos of hydrozoan systematists and marine naturalists of the past.
- http://www.ville-ge.ch/musinfo/mhng/hydrozoa/hydrozoa-directory.htm

Obelia colony
- Photo.
- http://www.ou.edu/class/invert/colony.htm