There are some fish species, such as flounder, that lack symmetry as adults. However, the larval fish are bilaterally symmetrical. Animals may be characterized by the presence of a coelom, formation of the mouth, and type of cell cleavage during embryonic development.
Most animal species undergo a separation of tissues into germ layers during embryonic development. Animals develop either two or three embryonic germs layers. Radially-symmetrical animals are diploblasts, developing two germ layers: an inner layer endoderm and an outer layer ectoderm.
Diploblasts have a non-living layer between the endoderm and ectoderm. Bilaterally-symmetrical animals are called triploblasts, developing three tissue layers: an inner layer endoderm , an outer layer ectoderm , and a middle layer mesoderm. Germ development in embryogenesis : During embryogenesis, diploblasts develop two embryonic germ layers: an ectoderm and an endoderm. Triploblasts develop a third layer, the mesoderm, between the endoderm and ectoderm. Each of the three germ layers in a blastula, or developing ball of cells, becomes particular body tissues and organs.
The endoderm gives rise to the stomach, intestines, liver, pancreas, and the lining of the digestive tract, as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract. The ectoderm develops into the outer epithelial covering of the body surface and the central nervous system.
The mesoderm, the third germ layer forming between the endoderm and ectoderm in triploblasts, gives rise to all muscle tissues including the cardiac tissues and muscles of the intestines , connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen.
Differentiation in triploblasts : Triploblasts may be a acoelomates, b eucoelomates, or c pseudocoelomates. Acoelomates have no body cavity. Eucoelomates have a body cavity within the mesoderm, called a coelom, which is lined with mesoderm. Pseudocoelomates also have a body cavity, but it is sandwiched between the endoderm and mesoderm.
Triploblasts that do not develop a coelom are called acoelomates: their mesoderm region is completely filled with tissue. Flatworms in the phylum Platyhelminthes are acoelomates.
Eucoelomates or coelomates have a true coelom that arises entirely within the mesoderm germ layer and is lined by an epithelial membrane.
This coelomic cavity represents a fluid-filled space that lies between the visceral organs and the body wall. It houses the digestive system, kidneys, reproductive organs, and heart, and it contains the circulatory system.
The epithelial membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates.
The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility and improved animal motility. The coelom also provides cushioning and shock absorption for the major organ systems, while allowing organs to move freely for optimal development and placement.
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