necto-, nect-, nekto-, nek- +
A nectosome is an order of Hydrozoa (various simple and compound polyps and jellyfish) consisting of various free-swimming or floating pelagic, mostly delicate and transparent, and often colored forms that are usually regarded as compound animals composed of zooids modified to perform various functions for the colony. Examples are feeding, defense, and locomotion which sometimes have two or more zooids in the form of a bell so that, by their contractions, they cause the colony to swim, and which often have a hollow float which keeps the colony afloat.
2. An organism that swims and is not simply carried passively by currents: Nektons are capable of counteracting currents through various swimming mechanisms.
Nekton are limited in distribution by temperature and nutrient supplies, and they decrease with the increasing depth of the sea.
Uncounted myriads of little plants, together with the great numbers of tiny animals that feed on them, and on each other, form the plankton.
The term "plankton" is the name applied in 1887 by Victor Hensen, a German professor of zoology, with reference to the great company of marine creatures that drift at the mercy of the currents, as distinguished from the nekton, animals like fish and whales which are able to swim against the moving waters, and from the "benthos", the plants and animals attached to the bottom of the sea or crawling on it.
There is no clear-cut dividing line between planktonic and nektonic creatures because some of the fish and other marine animals belong to the plankton during the early stages of their lives, and then to the nekton later.