The number of species of represented in the zooplankton of the sea is considerably enriched by the distinctive dispersal stages of many marine animals that spend their adult lives in the littoral or the benthos. To differing extents, these larvae (the amphiblastulae of sponges, the medusae and ephyrae of the cnidarian coelenterates, the pilidia of nemerteans, the trochospheres of polychaetes, the cypris larvae of cirripedes, the phyllosomae and zoeae of the eucarid malacostraca, the veligers of the lamellibranch mollusks, the various auriculariae, bipinnariae, and plutei of the echinoderms and the appendicularian larvae of the ascideans; (see Table II) share the diminutive size ranges, membranous translucence, and feeble swimming movements characteristic of the species which are planktonic throughout their lives. Among the smallest (20 mm, perhaps to 2 m). Though some of these are rather larger than some of the swimming organisms (“nekton”: fish, cephalopods) that are excluded from the understanding of “plankton” (discussed earlier), the large jellyfish qualify for their poor ability to control their own movements in the sea. The siphonophores, like Velella and the Portuguese man o'war, Physalis physalis, are little more than drifting “polyp colonies.” The true jelly-fish, which move themselves by slow, rhythmic pulsation of the umbrella-like manubrium, include the distinctive Aurelia, Cyanea, and Pelagia.

Finally, the young stages of several species of pelagic fish are of such diminutive size and swim so feebly and with weakness of motility that, for the first part of their lives, they are reasonably included among the plankton: Clupeids (herrings, sardines) and Scombrids (Mackerel) fulfill this description; ultimately demersal Gadids (cods and allies) and benthic flat fish (Pleuronectids, Soleids) also pass planktonic dispersal stages.