The structure of the body cavity influences movement

The body plans of triploblastic animals can be divided into three types based on the presence and structure of an internal, fluid-filled body cavity.

  1. Acoelomate animals such as flatworms lack an enclosed, fluid-filled body cavity. Instead, the space between the gut (derived from endoderm) and the muscular body wall (derived from mesoderm) is filled with masses of cells called mesenchyme (Figure 30.6A). These animals typically move by beating cilia.

  2. Pseudocoelomate animals have a body cavity called a pseudocoel, a fluid-filled space lying between the mesoderm and endoderm. Many of the internal organs are suspended in the pseudocoel, which is enclosed by muscles (mesoderm) only on its outside; there is no inner layer of mesoderm surrounding the internal organs (Figure 30.6B).

  3. Coelomate animals have a body cavity, the coelom, that develops within the mesoderm. The coelom is lined with a thin layer of tissue called the peritoneum, which also surrounds the internal organs. The coelom is thus completely enclosed by mesoderm (Figure 30.6C).

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Figure 30.6 Animal Body Cavities (A) Acoelomates do not have enclosed body cavities. (B) Pseudocoelomates have a body cavity enclosed by mesoderm only on its outside. (C) Coelomates have a body cavity that is enclosed by mesoderm on both its inside and its outside.

Activity 30.1 Animal Body Cavities

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The structure of an animal’s body cavity strongly influences the ways in which it can move. The body cavities of many animals function as hydrostatic skeletons. Fluids are relatively incompressible, so when the muscles surrounding a fluid-filled body cavity contract, fluids shift to another part of the cavity. If the body tissues around the cavity are flexible, fluids squeezed out of one region can cause some other region to expand. The moving fluids can thus move specific body parts. (You can see how a hydrostatic skeleton works by watching a snail emerge from its shell.) A coelomate animal has better control over the movement of the fluids in its body cavity than a pseudocoelomate animal does. An animal that has longitudinal muscles (running along the length of the body) as well as circular muscles (encircling the body cavity) has even greater control over its movement.

In terrestrial environments, the hydrostatic function of fluid-filled body cavities applies mostly to relatively small, soft-bodied organisms. Most larger animals (as well as many smaller ones) have hard skeletons that provide protection and facilitate movement. Muscles are attached to those firm structures, which may be inside the animal or on its outer surface (in the form of a shell or cuticle).