Bilateral Symmetry

Bilateral symmetry refers to the body plan where a plane or axis divides the entire body into two equal halves. The plane is called the sagittal plane or central axis, and the two halves are mirror images of each other. The back part of the body is the dorsal or the posterior end, while the front part is the ventral or the anterior end.

However, the internal body parts such as organs may not be symmetric. Higher organisms, like plants and animals, and lower invertebrates and microorganisms, like fungi and bacteria, all exhibit bilateral symmetry.

The term ‘bilateral’ is obtained from Latin ‘bis’ meaning ‘two’ and ‘latus’ meaning side. However, the word ‘symmetry’ came from Greek ‘syn’ means ‘together’ and ‘metron’ means ‘meter’.

99% of animals (belonging to phyla: Chordata, Annelida, Arthropoda, Platyzoa, Nematoda, and most Mollusca) and humans exhibit bilateral symmetry. Typical examples are dogs, cats, elephants, sharks, centipedes, and ants. A butterfly has an excellent display of bilateral symmetry. Its body is divided into symmetrical halves, and the patterns on each wing are almost identical. Marine organisms such as dolphins, sharks, sea turtles, fishes, lobsters, and echinoderms display bilateral symmetry. Humans have ears, arms, and legs that mirror images, making us bilaterally symmetrical.

The first simple multicellular organism that evolved with bilateral symmetry was called Bilateria, a sea anemone. Their ancestor probably appeared at the end of the Vendian period, the last geological period of the Neoproterozoic Era preceding the Cambrian Period, 635–541 million years ago. There has been an evolution of symmetry in animals after the emergence of bilateral symmetry, which later tends to become asymmetric with time.

Plants also developed bilateral symmetry but much later than animals.

Animals with bilateral symmetry:

• have only one line of symmetry
• have a head (anterior) and tail (posterior) region
• have a top (dorsal) and bottom (ventral) side
• have distinct left and right sides
• mainly have a complex brain, which is part of a developed nervous system and may even have right and left sides
• move more quickly than animals with radially symmetrical body plan
• and have better eyesight and hearing than those having radial symmetry

There are several advantages of bilateral symmetry, primarily in animals and humans.

Coordinating Body Parts

It helps the brain recognize and coordinate different body parts, making visual perception easier and causing better movement coordination.

Forming Body Polarity

It makes the body polar, where materials enter or leave from different directions instead of all happening through the same end.

Maintaining Body Balance

It is also essential to maintain body balance. Thus, it helps in the directional movement forward in a streamlined manner. For example, the two legs being equal in length helps us walk, which would have been impossible if they were of different lengths.

Escaping Predators

It helps animals evolve and move faster than their ancestors, allowing them to find food or escape predators.

Protection

Marine or freshwater organisms such as bivalve mollusks have shells consisting of two hinged, bilaterally symmetrical halves. The sagittal plane lies along the hinge, which allows the organism to firmly close its shell and protect the soft inner parts of the body.

Although a bilateral symmetry provides several benefits to an organism, its most significant disadvantage is that it reduces diversity in the structure and thus reduces the chance of evolution and adaptability.