Protostome

A member of one of the two major bilaterian lineages, including arthropods, mollusks, and annelids. Its name reflects the traditional, but not universal, pattern in which the embryonic blastopore contributes to the mouth.

Bilateria: The animal clade whose members develop bilateral symmetry and three primary germ layers. Protostomes form one of Bilateria’s two major branches.
Spiral cleavage: An embryonic cell-division pattern in which successive cleavage planes are oblique to the embryo’s axis. It occurs in many familiar protostomes, including mollusks and annelids.
Aristotle: An ancient Greek philosopher whose biological writings classified animals through observation. His comparative approach helped establish a long tradition of describing animal form.
Drosophila melanogaster: The fruit fly species widely used in genetics and developmental biology. This ecdysozoan model reveals conserved mechanisms of animal development.
Arthropod: An invertebrate animal with jointed appendages, a segmented body, and an external skeleton. Arthropods are the most species-rich protostome group.
Blastopore: The opening into the cavity formed during early embryonic gastrulation. Its developmental fate gave protostomes their traditional name.
Determinate cleavage: Early embryonic development in which a cell’s developmental fate becomes restricted relatively soon. This pattern characterizes many, but not all, protostome embryos.
Georges Cuvier: A French naturalist who organized animals into major body-plan groups in the early nineteenth century. His anatomical classification helped shape the historical framework for major animal divisions.
Caenorhabditis elegans: A nematode species used extensively to study development, genes, and nervous systems. Its transparent body and mapped cell lineage make protostome development experimentally tractable.
Mollusk: An animal in a diverse phylum that includes snails, clams, and cephalopods. Mollusks demonstrate how different body plans evolved within Protostomia.

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