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Bilateria (/ ˌ b aɪ l ə ˈ t ɪər i ə / BY-lə-TEER-ee-ə) [5] is a large clade or infrakingdom of animals called bilaterians (/ ˌ b aɪ l ə ˈ t ɪər i ə n / BY-lə-TEER-ee-ən), [6] characterized by bilateral symmetry (i.e. having a left and a right side that are mirror images of each other) during embryonic development.
Echinoderms evolved from animals with bilateral symmetry. Although adult echinoderms possess pentaradial symmetry, their larvae are ciliated, free-swimming organisms with bilateral symmetry. Later, during metamorphosis, the left side of the body grows at the expense of the right side, which is eventually absorbed.
Arthropoda is the phylum with the most individual organisms. Bilateria is an extremely diverse group of animals containing a vast majority of its species, largely due to the enormous amount of arthropods. This article is a list of orders contained within Bilateria separated by phylum. Groups that are not contained within an order are listed ...
Like many other microscopic animals, adult rotifers frequently exhibit eutely—they have a fixed number of cells within a species, usually on the order of 1,000. Bdelloid rotifer genomes contain two or more divergent copies of each gene, suggesting a long-term asexual evolutionary history. [21] For example, four copies of hsp82 are found.
All bilaterians have some asymmetrical features; for example, the human heart and liver are positioned asymmetrically despite the body having external bilateral symmetry. [14] The bilateral symmetry of bilaterians is a complex trait which develops due to the expression of many genes. The Bilateria have two axes of polarity.
The phylum Arthropoda contains numerous taxonomic orders in over 20 classes. Subphylum Hexapoda Order ... Order Decapoda (Crayfish, crabs, lobsters, prawns, ...
Echinoderms are unique among animals in having bilateral symmetry at the larval stage, but fivefold symmetry (pentamerism, a special type of radial symmetry) as adults. [55] The echinoderms are important both biologically and geologically.
Cephalization is a characteristic feature of the bilaterians, a large group containing the majority of animal phyla. [3] These have the ability to move, using muscles, and a body plan with a front end that encounters stimuli first as the animal moves forwards, and accordingly has evolved to contain many of the body's sense organs, able to detect light, chemicals, and gravity.