Animals are eukaryotic, multicellular organisms that form the biological kingdom Animalia. With few exceptions, animals are motile (able to move), heterotrophic (consume organic material); they reproduce sexually, and their embryonic development includes a blastula stage. The body plan of the animal derives from this blastula, differentiating specialized tissues and organs as it develops; this plan eventually becomes fixed, although some undergo metamorphosis at some stage in their lives. Zoology is the study of animals. Over 66 thousand vertebrate and over 1.3 million invertebrate species currently exist. Classification of animals into groups (taxonomy) is accomplished using either the hierarchical Linnaean system; or cladistics, which displays diagrams (phylogenetic trees) called cladograms to show relationships based on the evolutionary principle of the most recent common ancestor. Some recent classifications based on modern cladistics have explicitly abandoned the term "kingdom", noting that the traditional kingdoms are not monophyletic, i.e., do not consist of all the descendants of a common ancestor. Body plans are helpful in their classification. Animals can be divided broadly into vertebrates and invertebrates. Vertebrates—fishes, amphibians, reptiles, birds, and mammals—have a backbone or spine (vertebral column), and amount to less than five percent of all described animal species. All vertebrate species and most invertebrates—arthropods, molluscs, roundworms, ringed worms, flatworms, and other phyla in Ecdysozoa and Spiralia—are bilaterally symmetric. Echinoderm larvae are bilaterally symmetrical, although they develop into radially symmetrical adults. Cnidarians are radially symmetric, while ctenophores are biradially symmetric. Sponges have no symmetry. Animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. Animals emerged as a clade within Apoikozoa as the sister group to the choanoflagellates.