Question

Most of the sponges are (a) Bilateral (b) Radial (c) Biradial (d) Asymmetric

Short answer

The most appropriate classification of sponges based on their symmetry is (d) Asymmetric.

Step-by-step solution

Understanding the Terms

In this step, an understanding of each term is important. Bilateral symmetry is a central property of an object that can be divided into two identical halves by a single plane. Radial symmetry refers to the arrangement of body parts around a central axis. Biradial symmetry is a combination of bilateral and radial symmetry, where the animal can be divided into identical halves by more than one plane passing through the central axis. Asymmetric indicates no symmetry.

Classify Sponges Based on Symmetry

Sponges or Porifera, are simple multicellular organisms that lack any definite symmetry. This means that they could not be divided into two identical halves by any plane passing through the central axis.

Key concepts

Porifera

The phylum Porifera, commonly known as sponges, represents some of the simplest multicellular organisms found in aquatic environments. Whether dwelling in freshwater or marine settings, these organisms boast a porous body structure, which is how they received their name—'Porifera' originates from Latin, meaning 'bearing pores'.

The anatomy of a sponge is quite distinct. They lack true tissues and organs seen in more complex life forms. Instead, sponges are composed of a loose aggregation of cells that perform various functions such as filtering nutrients from the water, removing waste, and supporting the sponge's structure with tiny skeletal elements called spicules. The sponges' porous body aids in their primary function: filter feeding. Water flows through the small openings (pores) into a central cavity and exits through a larger hole known as the osculum.

Reproduction in sponges can be both asexual, through budding or fragmentation, and sexual, by releasing sperm cells into the water which are then taken up by other sponges with the help of their water current system.

Bilateral Symmetry

Bilateral symmetry is a form of symmetry where an organism can be divided into two mirror-image halves along only one plane, which bisects the organism through its midline. Common in animals, this type of symmetry is associated with a preferred direction of movement, typically forward, which favors the development of a head (cephalization) and sensory organs concentrated at the front end.

Organisms with bilateral symmetry tend to exhibit complex behaviors and advanced nervous systems compared to those with other forms of symmetry. Examples are vertebrates like mammals, birds, fish, and even invertebrates such as insects and worms. This structure is evolutionarily advantageous for actively mobile animals, as it allows for directional movement and a streamlined body shape.

Radial Symmetry

Radial symmetry defines a body plan in which any number of imaginary planes, drawn through the central axis of an organism, divide it into identical halves. This type of symmetry is typical in organisms that are sessile (non-movable) or planktonic (drifting), such as many types of jellyfish, sea anemones, and adult starfish.

The advantage of radial symmetry is that it enables these organisms to interact with their environment from all sides. This is particularly beneficial for stationary species, as they can receive stimuli and capture food from any direction. In marine environments, where conditions can be uniform, radial symmetry is a favorable adaptation for survival.

Asymmetry

Asymmetry in biology implies the absence of any plane of symmetry, meaning that no matter how the organism is divided, there are no two identical halves. Asymmetry is much rarer in animals but is most commonly seen in Porifera, the sponges.

Sponges' asymmetrical body was once thought to be an evolutionary disadvantage. However, this irregular structure has its benefits, such as providing a sturdy foundation in various substrates and a unique approach to filter feeding that doesn't rely on symmetry. The lack of symmetry in sponges results from their highly adaptive growth patterns, allowing them to maximize their feeding efficiency and adapting to irregularities in their immediate environment.