Protostomia

Protostomia is a major clade within the Bilateria that includes the majority of invertebrate animals. Along with Deuterostomia, it forms one of the two principal divisions of bilaterian animals. Protostomes are defined largely by embryological features: early cleavage is usually spiral and determinate, and the blastopore commonly forms the mouth, with the anus arising from a secondary opening later in development. The coelom is typically formed by schizocoely, although there are exceptions in which coelomic formation varies among different lineages.

The clade encompasses two large groups, the Lophotrochozoa and the Ecdysozoa, which together include many familiar phyla such as Arthropoda, Mollusca, and Annelida, as well as Nematoda, Nemertea, and numerous lesser-known lineages. Protostomes exhibit extensive ecological and morphological diversity, ranging from segmented worms to shell-bearing mollusks and countless exoskeleton-bearing arthropods. Larval forms such as trochophore-like larvae occur in several lineages, though not universally.

In contrast, Deuterostomia is typically characterized by radial and indeterminate cleavage and by the anus forming from the blastopore with the mouth arising secondarily. Molecular phylogenies support Protostomia as a coherent evolutionary lineage within the Bilateria, reflecting a shared developmental toolkit that has driven vast diversification across terrestrial, freshwater, and marine environments.