Question

The Wikipedia article for Symmetry in Biology claims: Animals with bilateral symmetry are classified into a large group called the Bilateria which contains 99% of all animals (comprising over 32 phyla and 1 million described species). This claim that almost all animals are Bilateria can be found in other sources too, e.g. in A new paradigm for animal symmetry by Gábor Holló. In both cases, however, no source is provided. I'm no biologist so I don't no exactly how self-evident this claim is.

Does someone know a source corroborating these claims?

Answer 1

In biology, most animals fall under the Bilateria clade, characterized by bilateral symmetry, which is correlated with higher organismal complexity and mobility. Bilateral symmetry distinguishes these animals from those with radial or asymmetrical body plans and is associated with a triploblastic body structure and typically, a coelom. Such organization is prevalent amongst animal diversity, supporting the claim that Bilateria contains the majority of animal species.

Step-by-step explanation:

Animal body plans and classification have traditionally been centered around the concept of body symmetry. Bilateral symmetry, as observed in the Bilateria clade, includes a wide range of animals, representing the vast majority of animal phyla. This symmetry allows for a separation into mirror-image right and left sides when an organism is bisected along a single plane. In Bilateria, we find advanced traits such as cephalization, which is the development of a head region where nervous tissues are concentrated, facilitating more complex behaviors and movement. In contrast, radial symmetry, common in aquatic and sessile organisms, allows for an organism to be divided into equal halves along multiple planes, which is conducive to a lifestyle with less directed mobility.

Radial symmetry is found in phyla like Cnidaria and often involves organisms with two germ layers, ectoderm and endoderm. In comparison, bilaterally symmetrical animals typically have three germ layers (making them triploblasts), including a mesoderm, and may develop a body cavity, or coelom. Having a coelom provides physiological benefits and is an evolutionary advantage that is a defining feature of more complex organisms. Furthermore, coelomates can be subdivided into protostomes and deuterostomes based on various developmental criteria.

Echinoderms, such as sea stars and sea urchins, display secondary radial symmetry as adults, but as larvae, they exhibit bilateral symmetry, highlighting an evolutionary adaptation. The Eumetazoa, or 'true animals', consist of those with radial and bilateral symmetry, with the latter encompassing most animal diversity, consistent with the Bilateria claims. While direct sources confirming the exact percentage of animals falling under Bilateria are not always cited, the consensus leans towards an overwhelming majority based on established classifications and evolutionary biology.