Non-bilaterians
Sponges are physically very distinct from other animals, and were long thought to have diverged first, representing the oldest animal phylum and forming a sister clade to all other animals.[120] Despite their morphological dissimilarity with all other animals, genetic evidence suggests sponges may be more closely related to other animals than the comb jellies are.[121][122] Sponges lack the complex organisation found in most other animal phyla;[123] their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals.[124] They typically feed by drawing in water through pores, filtering out small particles of food.[125]
The Ctenophora and Cnidaria are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus.[126] Animals in both phyla have distinct tissues, but these are not organised into discrete organs.[127] They are diploblastic, having only two main germ layers, ectoderm and endoderm.[128]
The tiny placozoans have no permanent digestive chamber and no symmetry; they superficially resemble amoebae.[129][130] Their phylogeny is poorly defined, and under active research.[121][131]