3 The Acoelomate-Coelomate Split
Triploblastic animals with solid bodies-that is, without a cavity between the gut and outer body wall-are referred to as the acoelomates (Gr.a,”without,” and koilos,”a hollow”). This group includes flatworms (phylum Platyhelminthes) and animals of a few other phyla (FIGURE 29.4a). In contrast to these acoelomates, most phyla of bilateral, triploblastic animals have tube-within-a-tube body plans, with a fluid-filled cavity separating the digestive tract from the outer body wall. There is a second important difference between acoelomates and animals having a body cavity: Animals with a body cavity also have some form of blood vascular system-blood that is circulated through a network of spaces or vessels; acoelomates lack a blood vascular system.
3 The Acoelomate-Coelomate SplitTriploblastic animals with solid bodies-that is, without a cavity between the gut and outer body wall-are referred to as the acoelomates (Gr.a,”without,” and koilos,”a hollow”). This group includes flatworms (phylum Platyhelminthes) and animals of a few other phyla (FIGURE 29.4a). In contrast to these acoelomates, most phyla of bilateral, triploblastic animals have tube-within-a-tube body plans, with a fluid-filled cavity separating the digestive tract from the outer body wall. There is a second important difference between acoelomates and animals having a body cavity: Animals with a body cavity also have some form of blood vascular system-blood that is circulated through a network of spaces or vessels; acoelomates lack a blood vascular system.
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