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A coelom is a body cavity lined with mesoderm surrounding the digestive organ or system. The term acoelomate means that the organism doesn t have a body cavity and the mesoderm is packed inbetween the ectoderm and the endoderm. Pseudocoelomates have a body cavity that is incompletely lined with mesoderm so their organs lie loose.
In protostomes, the mouth develops near the blastospore first, and then the anus forms. In deuterostomes however, the anus develops near the blastospore and the mouth forms second. Cleavage is the first event in development. In protostomes, spiral cleavage occurs, and daughter cells sit in grooves formed by previous cleavages. The fate of these cells is fixed in protostomes, which means each can contribute to development in only one way. In deuterostoms, radial cleavage occurs, and daughter cells sit on top of the previous cells. The fate of these cells is indeterminat, which means that if they are seperated from each other, then each cell can go on to become a complete organism.
Radial symmetry means that if you cut along the center anywhere on the organism, the organism will be simmilar on both sides. Bilateral symmetry means that you can only cut down the length of the organism and have the two halves be simmilar.
A phylogenetic tree is a diagram that indicates common ancestors and lines of descent, and embryonic development. Classification of animals is based on type of symmetry, number of tissue layers, type of coelom, and presence of segmentation. The bottom half of the plylogenetic tree contains organisms that lack a true coelom such as the porifera, cnideria, platyhelminthes. The rotifera and nematoda are pseudocoelomates which are right above the acoelomates. Everything above are coelomates, wich are either protostomes or deuterostomes. This allow seperation of echinodermata and chordata, which are deutrostomes, and arthropods, mollusca, and annelida, which are protostomes.
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