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# Biology Symmetry Help

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## Orderly Patterns Of Body Form Within Invertebrates

Besides lacking a backbone or vertebral column, the invertebrates display other orderly patterns of body form. One fundamental pattern is symmetry ( SIM -et-ree). The term, symmetry, exactly translates from the Ancient Greek to mean, “the process of measuring together.” In modern times, however, we are not really referring to measuring anything (as with a ruler). Rather, we are carefully looking at the shape and size of two things, compared to each other. Symmetry is said to be present, then, whenever there is a rough balance or equality of body shape and size, on either side of some dividing line. Since invertebrates lack a stiffening backbone, their bodies tend to be much more flexible than those of vertebrates. Thus, symmetry has become an important organizing influence or pattern for their survival.

### Bilateral Symmetry (Mirror-Image)

There are several specific kinds of symmetry (see Figure 10.2). The most familiar to most people is bilateral (buy- LAT -er-al) or mirror-image symmetry. In this type of symmetry, an imaginary line subdivides the body into two equal halves. The right half of the body is then considered a mirror image of the left half of the body. Consider, for example, a line drawn lengthwise through the middle of a lobster (Figure 10.2, A). The lobster body often has a high degree of bilateral (mirror-image) symmetry, because the right side of the body is a mirror image of the left side, and vice versa. (By mirror image, it is meant that both sides of the body have the same shape and size, but that right and left are reversed.)

Fig. 10.2 Two kinds of symmetry in eumetazoans. (A) Bilateria (bilateral symmetry). (B) Radiolarians (radial symmetry).

[ Study suggestion: Get up out of your chair and go look at your reflection in a full-length mirror. To what degree does your body and its reflection show the characteristic of bilateral or mirror image symmetry?] An overhead view of an automobile also often reveals a high degree of bilateral (mirror image) symmetry.

Another important type of rough balance is called radial ( RAY -dee-al) symmetry . In radial symmetry, there is a rough balance of various parts or “rays” ( radi ) that come out from the same center or axis. [ Study suggestion: Picture the sun and its rays, which make a radial symmetry.] Consider, for example, the identical tendrils or arms of a jellyfish, which seem to radiate ( RAY -dee- ate ) out from a central axis in the middle of its body (Figure 10.2, B). The jellyfish, therefore, has radial symmetry. This makes it somewhat resemble a wheel with a central hub, around which a series of spokes radiate.

## Anatomy Of The Bilateria Versus The Radiolarians

### Bilateria

Animals with a bilateral symmetry body plan are technically called the bilateria ( buy -lah- TEER - ee-uh), or “two-sided” animals. The bilateria (bilateral animals) include most vertebrates, as well as many invertebrates. They have more to their body plan than just left and right sides. Bilateria have a head or anterior (an- TEER -ee-or) end, that lies in “front” ( anteri -). And they have a tail or posterior (pahs- TEER -ee-or) end, that follows “behind” ( posteri -). Since the bilateria have a “head” or cephalic (seh- FAL -ik) end to their bodies, we say that they show the characteristic called cephalization ( sef -uh-luh- ZAY -shun). By cephalization, it is meant that an animal has a definite head end to its body, usually containing the main collection of its sensory organs (such as the brain and eyes and sound detectors).

Further, the bilateria have an upper or dorsal ( DOOR -sal) side in their “back” ( dors ), as well as a lower or ventral ( VEN -tral) side on their “belly” ( ventr ).

Another main group of invertebrates are the radiolarians ( ray -dee-oh- LAIR -ee-uns) – creatures with “little rays” ( radiol ) or spines projecting out-ward from their bodies. The jellyfish with its many radiating arms, of course, is a typical radiolarian. In these animals, there is no head or rear end, nor left or right side. They do not show the characteristic of cephalization. There is, however, both a superior ( soo-PEER -e-or) portion of the animal lying “above” ( superi ) most of the body, and an inferior ( in-FEER -e-or) portion lying “below” ( inferi ). (Review Figure 10.2 to see these terms of relative body position.)

Fig. 10.2 Two kinds of symmetry in eumetazoans. (A) Bilateria (bilateral symmetry). (B) Radiolarians (radial symmetry).

Practice problems for these concepts can be found at: Invertebrates Test

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