natantia

Definitions

n. The free rotifers: opposed to Sessilia.
n. In Illiger's classification of mammals (1811), the fourteenth order, containing the sirenians and cetaceans as two families, Sirenia and Cete: same as Mutilata.
n. In conch.:
n. A division of azygobranchiate gastropods, containing the natant or free-swimming oceanic or pelagic forms usually called heteropods, and corresponding to the class or order Heteropoda.: opposed to Reptantia.
n. A section of cephalate mollusks proposed for the cephalopods.
n. A suborder of peritriehous ciliate infusorians, containing those which are free-swimming: opposed to Sedentaria.

Sorry, no etymologies found.

Olim in Oceano Australi, ubi rectis "oculis monstra natantia" vidit, victoriam prope primam, velut alter Perseus, a Medusa reportavit; varias deinceps animantium formas quasi ab ipsa Gorgone in saxum versas sagacitate singulari explicavit; vitae denique universae explorandae vitam suam totam dedicavit.

Life and Letters of Thomas Henry Huxley — Volume 2
The praecipitem Africum decertantem Aquilonibus, et rabiem Noti and the monstra natantia of the Adriatic, are somewhat enlarged; but Horace trembling for the life of Virgil, is an interesting moment in the history of poetry and friendship.] [Footnote 68: (Alexias, l.iv. p. 106.)

History of the Decline and Fall of the Roman Empire — Volume 5
The praecipitem Africum decertantem Aquilonibus, et rabiem Noti and the monstra natantia of the Adriatic, are somewhat enlarged; but Horace trembling for the life of Virgil, is an interesting moment in the history of poetry and friendship.] 68 (Alexias, l.iv. p. 106.)

The History of the Decline and Fall of the Roman Empire