Treated as plural. A former major grouping of the animal kingdom including invertebrates with no distinct nervous system.
((n. pl.) The lowest groups of animals, in which no nervous
system has been observed.)
Origin:
Early 19th century; earliest use found in Annals of Philosophy. From scientific Latin Acrita from ancient Greek ἄκριτος undistinguishable, undecided, doubtful, not giving judgement + scientific Latin -a.