Head of a hydroid
© Getty
Hydroids are invertebrates that either live
alone or form colonies in freshwater
environments. There are around 3,700
known species of hydroids around the
world. During their lives, hydroids enter
into three stages of life. This image shows
the head of a hydroid (Ectopleura larynx)
during its polyp stage with its tentacles
unfurled, which house toxic cells to
catch and kill plankton prey.