A coelom is a fluid-filled body cavity that arises from within mesoderm tissue early in embryonic development. It is a significant advance in the course of animal evolution because it provides space for elaborate body systems like a transport system. More advanced phyla, such as chordate and echinodermata have a coelom. An animal that does not have a coelom, an acoelomate, lacks internal cavities and complex organs. Primitive acoelomate phyla are Cnidaria, Porifera, and Platyhelminthes. Some animals have a pseudocoelom, an internal cavity that is only partly lined with mesoderm tissue. An animal phylum with a pseudocoelom is the Nematodoa.
Protostome development is a developmental mode in animals distinguished by the development of the mouth from the blastopore; often also characterized by spiral cleavage and by the body cavity forming when solid masses of mesoderm split. In protostome development, the anus develops later. Deuterostome development is a developmental mode distinguished by the development of the anus from the blastopore and often also characterized by radial cleavage and by the body cavity forming as outpockets of mesodermal tissue. In this form of development, the mouth forms secondarily.
Radial symmetry is where several planes can pass through the long axis and divide an animal into similar parts. An example of an animal with radial symmetry is the hydra. If only one plane can bisect the animal into left and right halves, the animal has bilateral symmetry. Primitive organisms show radial symmetry, whereas more advanced organisms show bilateral symmetry. Chordates all have bilateral symmetry. The embryos of echinoderms demonstrate bilateral symmetry but revert to the primitive radial symmetry as adults.
Phylogenetic trees base taxonomic relationships on homologous structures, patterns of embryonic development, and common ancestry. There is great diversity in the animal kingdom, and they can be grouped by several characteristics. These...
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