Животные /english/

All are marine.Reproduction is predominantly sexual, with the embryo passing through a blastula phase .The latter type, protozoa, was categorized as a subkingdom of Animalia.What constitutes an animal, therefore, depends on the scheme followed.One basic body part is the gut, which must have arisen early in the evolution of the metazoans.Furthermore, they have tissues but no real organs or nerves, and they lack symmetry.A nervous system is present, but no brain or head.Instead, the animals exhibit radial symmetry, or symmetry around a central axis.The tissues of the Radiata have only two major layers, whereas the Bilateria have three.Such groups are founded on the basis of how the cells divide and how various organs are formed.