A deuterostome represents a significant classification within the animal kingdom, defined by specific patterns observed during early embryonic development. This categorization helps scientists understand the evolutionary relationships among diverse animal species. The way an organism develops from a fertilized egg provides fundamental insights into its biological blueprint.
Understanding Deuterostomes
The term “deuterostome” originates from Greek, meaning “mouth second.” This refers to a defining characteristic of their embryonic development: the anus forms from the blastopore (the first opening during gastrulation), and the mouth develops later at a secondary opening. This developmental sequence groups various animals into the superphylum Deuterostomia.
Key Developmental Features
Deuterostomes exhibit several distinct developmental characteristics. Their embryonic cells undergo radial cleavage, where cell divisions occur parallel or perpendicular to the embryo’s polar axis, creating a symmetrical arrangement.
This radial cleavage is indeterminate, meaning the developmental fate of individual cells is not rigidly determined in the earliest stages. If embryonic cells are separated early, each cell retains the potential to develop into a complete organism, allowing for the formation of identical twins in humans.
The coelom, the main body cavity, forms through enterocoelous coelom formation. During this process, the coelom arises from out-pocketings or pouches that bud off from the archenteron (the primitive gut of the embryo). These pouches expand and eventually fuse to form the coelomic cavity.
Animals That Are Deuterostomes
The deuterostome group encompasses several major animal phyla, including some of the most complex life forms. Chordates, a diverse phylum that includes all vertebrates like fish, amphibians, reptiles, birds, and mammals (including humans), are prominent examples.
Echinoderms, a phylum of marine invertebrates, also belong to this group, with familiar examples including sea stars, sea urchins, and sea cucumbers. While adult echinoderms often display radial symmetry, their larval stages exhibit bilateral symmetry, reflecting their deuterostome lineage. Additionally, hemichordates, such as acorn worms, are another phylum within the deuterostome superphylum.
Deuterostomes Compared to Other Life Forms
Deuterostomes are contrasted with protostomes, the other major group of bilaterally symmetrical animals. These two groups are primarily distinguished by fundamental differences in their early embryonic development. In protostomes, the blastopore (the first embryonic opening) develops into the mouth, and the anus forms secondarily.
Protostomes also exhibit spiral cleavage, where cells divide at oblique angles, leading to a non-aligned arrangement of cells. Their cleavage is determinate, meaning the developmental fate of each embryonic cell is set very early. The coelom in protostomes typically forms through schizocoely, a process involving the splitting of mesodermal tissue. Many familiar invertebrates, such as insects, mollusks, and annelids, are classified as protostomes.