A ball of corethron via The Microbial Loop (Ice Stories: Dispatches from Polar Scientists)

Plumose anemone (Metridium)

By Vidar A from Gozo, Malta (Pulmose anemone) [CC-BY-2.0], via Wikimedia Commons

Diatoms, microscopic beauty.

Source (Midlands Technical College)

Bdelloid rotifers (microscopic freshwater invertebrates) (20x, Technique: Polarized Light) by Michael Shribak & Dr. Irina Arkhipova, Marine Biological Laboratory Woods Hole, Massachusetts, USA.

The Nikon International Small World Competition (link)

Porpita porpita has a small disc like body and floats freely in the water column. Related to the jellyfish, this species measures just one inch in diameter.

Image courtesy of Islands in the Sea 2002, NOAA/OER.

Source: NOAA Ocean Explorer