What Characteristics Define and Distinguish Animals?
5. Animal cells secrete an extracellular matrix. 6. Animals are heterotrophs and may be carnivores, herbivores, or detritivores. 7. Animal bodies exhibit degrees of organization into tissues, organs, and organ systems. 8. Body symmetry may be radial with an oral end and aboral end, or bilateral, with a cephalized (head) end. 9. An animal zygote undergoes cleavage divisions to form a blastula and then a gastrula, which has ectoderm, endoderm, and in some species, mesoderm. Those with two layers are diploblastic; with three layers, triploblastic. 10. Animals may undergo direct development or metamorphose from a larva to an adult. They reproduce sexually and sometimes asexually. 11. A coelom is a body cavity lined with mesoderm. Animals may be coelomate, acoelomate, or pseudocoelomate. 12. Major coelomate lineages differ in developmental pattern. Protostomes have spiral and determinate cleavage, and a blastopore developing into a mouth. Deuterostomes have radial and indeterminate cleavage
