Body Plans Basic Structural Designs

The general structure of an animal, its organ systems, and the integrated functioning of its parts are known as its body plan. A fundamental aspect of an animal's body plan is its overall shape, described in part by its symmetry. A symmetrical animal can be divided along at least one plane into similar halves. Animals that have no plane of symmetry are said to be asymmetrical. Many sponges are asymmetrical, but most animals have some kind of symmetry.

The simplest form of symmetry is spherical symmetry, in which body parts radiate out from a central point. An infinite number of planes passing through the central point can divide a spherically symmetrical organism into similar halves. Spherical symmetry is widespread among the pro-tists, but most animals possess other forms of symmetry.

An organism with radial symmetry has one main axis around which its body parts are arranged. A perfectly radially symmetrical animal can be divided into similar halves by any plane that contains the main axis. Some simple sponges and a few other animals, such as sea anemones (Figure 32.2a), have radial symmetry. Most radially symmetrical animals are slightly modified so that fewer planes can divide them into identical halves. Two animal phyla—Cnidaria and Ctenophora—are composed primarily of radially symmetrical animals. These animals move slowly or not at all.

Bilateral symmetry is a common characteristic of animals that move rapidly through their environments. A bilaterally symmetrical animal can be divided into mirror images (left and right sides) by a single plane that passes through the dorsoventral midline of its body from the front (anterior) to the back (posterior) end (Figure 32.2b). A plane at right angles to the first one divides the body into two dissimilar sides; the back side of a bilaterally symmetrical animal is its dorsal surface; the belly side is its ventral surface.

The sea anemone, a cnidarian, has radial symmetry.

Any plane along the main body axis divides the animal into similar halves.

The sea anemone, a cnidarian, has radial symmetry.

Any plane along the main body axis divides the animal into similar halves.

Cnidarians Symmetry

The fish, a vertebrate, has bilateral symmetry.

The fish, a vertebrate, has bilateral symmetry.

Bilateral Symmetry Fish
Only one plane divides the animal into similar, mirror-image halves.

32.2 Body Symmetry symmetrical.

Most animals are either radially or bilaterally

Bilateral symmetry is strongly correlated with cephaliza-tion: the concentration of sensory organs and nervous tissues in a head at the anterior end of the animal. Cephalization is favored because the anterior end of a freely moving animal typically encounters new environments first.

Fluid-filled spaces, called body cavities, lie between the ectoderm and endoderm of most protostomes and deuteros-tomes. The type of body cavity an animal has strongly influences the way it moves.

► Animals that lack an enclosed body cavity are called acoelomates. In these animals, the space between the gut and the body wall is filled with masses of cells called mesenchyme (Figure 32.3a).

► Pseudocoelomate animals have a body cavity called a pseudocoel, a liquid-filled space in which many of the internal organs are suspended. Their control over body shape is crude because the pseudocoel has muscles only on its outside; there is no inner layer of muscle surrounding the organs (Figure 32.3b).

► Coelomate animals have a coelom, a body cavity that develops within the mesoderm. It is lined with a special structure called the peritoneum and is enclosed on both the inside and the outside by muscles (Figure 32.3c).

The fluid-filled body cavities of simple animals function as hydrostatic skeletons. Because fluids are relatively incompressible, they move to another part of the cavity when the muscles surrounding them contract. If the body tissues around the cavity are flexible, fluids squeezed out of one region can cause some other region to expand. The moving flu-

32.3 Animal Body Cavities There are three major types of body cavities among the animals. (a) Acoelomates do not have enclosed body cavities. (b) Pseudocoelomates have only one layer of muscle, and it lies outside the body cavity. (c) Coelomates have a peritoneum surrounding the internal organs.The body cavities of some coelomates, such as this earthworm, are segmented.

Gut Muscle layer

(endoderm) (mesoderm)

(b) Pseudocoelomate

Flatworm

Roundworm

Gut Muscle layer

(endoderm) (mesoderm)

Flatworm Gut Endoderm

Mesenchyme

Acoelomates do not have enclosed body cavities.

Ectoderm Gut (endoderm)

Pseudocoel

Muscle (mesoderm)

Internal organs

Mesenchyme

Ectoderm

(b) Pseudocoelomate

Roundworm

Ectoderm

Pseudocoelomate

Pseudocoelomates have a cavity lined with mesoderm on the outer side, but no mesoderm surrounds the internal organs.

ids can thus move specific body parts. If a temporary attachment can be made to the substratum, the whole animals can move from one place to another.

In animals that have both circular muscles (encircling the body) and longitudinal muscles (running along the length of the body), the action of these antagonistic muscles on the fluid-filled body cavity gives the animal even greater control over its movement. A coelomate animal has better control over the movement of the fluids in its body cavity than does a pseudocoelomate animal, but its control is further improved if the coelom is separated into compartments or segments. Then muscles in each individual segment can change its shape independently of the other segments. Segmentation of the coelom evolved several different times among both protostomes and deuterostomes.

Other forms of skeletons developed in many animal lineages, either as substitutes for, or in combination with, hydrostatic skeletons. Some skeletons are internal (such as vertebrate bones); others are external (such as lobster shells). Some external skeletons consist of a single element (snail shells), others have two elements (clam shells), and still others have many elements (centipedes).

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  • barbara
    How does an animal's symmetry influences the way it moves?
    8 years ago
  • claudia
    How does an animals symmetry influences the way it moves.?
    8 years ago
  • pervinca
    How does an animal symmetry influences the way it move?
    8 years ago
  • Prospero
    How an animal symmetery influences the way it moves?
    8 years ago
  • fredrik
    How does animal symmentry influences the way it moves?
    8 years ago
  • ky
    How does an animal symmetry influence the way it moves?
    8 years ago
  • jonas
    Do pseudocoelomates lack a muscle layer around the gut?
    8 years ago
  • lola
    How body cavities influence animals?
    8 years ago
  • Is the symmetry of a sea anemone radial?
    8 years ago
  • medhane
    Are fish bilateral symmetry?
    7 years ago

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