Obelia hydrozoan medusa. Light micrograph (LM) of a medusa (young polyp) from the Obelia geniculata hydroid. The circular shallow semi-bell has solid tentacles on the rim. At the center, is the manubrium, with oral tentacles for ingesting food. The four dark round areas are the gonads. The gonads on a single medusa are all the same sex, male or female. If the medusa is female, the eggs will be fertilised by sperm from another medusa via the sea, and the fertilised medusa will then develop into another hydroid colony. Magnification: x14 when printed at 10 centimetres across.