Protostomes undergo spiral cleavage, while deuterostomes undergo radial cleavage.In protostomes, the coelom forms when the mesoderm splits through the process of schizocoely, while in deuterostomes, the coelom forms when the mesoderm pinches off through the process of enterocoely.Bilaterally symmetrical, tribloblastic eucoelomates can be divided into protostomes, those animals that develop a mouth first, and deuterstomes, those animals that develop an anus first and a mouth second.
Triploblasts can be further categorized into those without a coelom ( acoelomates ), those with a true coelom (eucoelomates), and those with “false” coeloms ( pseudocoelomates ).Protostomes include arthropods, mollusks, and annelids. In Deuterostomes (mouth second), the mouth develops at the other end of the gut (Figure 3) and the anus develops at the site of the blastopore. The endoderm becomes the digestive and respiratory tracts the ectoderm becomes the outer epithelial covering of the body surface and the central nervous system and the mesoderm becomes all muscle tissues, connective tissues, and most other organs. In Protostomes (mouth first), the mouth develops at the blastopore (Figure 3).
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