Animals are sorted into phyla not by a single trait but by a fixed sequence of body-plan criteria, each of which splits the animal kingdom into large, natural groups before finer distinctions are made. The first criterion is the level of organisation. In sponges, cells are arranged as loose aggregates with no true tissues (cellular level); in coelenterates, cells performing the same function combine into tissues (tissue level); in Platyhelminthes, tissues combine into organs (organ level); and from Annelida onward, organs work together as organ systems (organ-system level). Independent of this hierarchy, a digestive system may be a blind sac with a single opening (incomplete, as in Platyhelminthes and Coelenterata) or a tube with a separate mouth and anus (complete, as in annelids onward), while a circulatory system may be open, with blood not always confined to vessels (arthropods), or closed, remaining within vessels throughout (annelids, chordates).
The second criterion is symmetry: sponges are largely asymmetrical (no plane divides them into equal halves); coelenterates, ctenophores and adult echinoderms are radially symmetrical (any plane through the central axis gives two equal halves); and Platyhelminthes onward are bilaterally symmetrical (only one plane, through the long axis, gives identical left and right halves).
The third criterion is the number of embryonic (germ) layers: coelenterates are diploblastic, with an outer ectoderm and inner endoderm separated by an undifferentiated mesoglea, while Platyhelminthes onward are triploblastic, adding a true mesoderm between the two.
The mesoderm's arrangement then defines the coelom: a coelomate has a body cavity fully lined by mesoderm (annelids, molluscs, arthropods, echinoderms, chordates); a pseudocoelomate has a body cavity that is not mesoderm-lined (Aschelminthes); an acoelomate has no body cavity at all (Platyhelminthes). Later criteria include segmentation (serial repetition of body segments, from Annelida onward) and the presence of a notochord, which marks the chordates off from every other phylum in the animal kingdom.
NEET frequently blurs pseudocoelomates with acoelomates: a pseudocoelomate such as Ascaris does have a fluid-filled body cavity; it is simply not lined by mesoderm, unlike a true coelomate. Only the acoelomate flatworms genuinely lack any body cavity at all.