Protostome

Protostomes are a major lineage of animals in which the mouth forms from or near the first opening of the embryo, the blastopore, during early development. In contrast, deuterostomes typically form the anus from the blastopore and later develop the mouth. Protostomes comprise two large clades, the Lophotrochozoa and the Ecdysozoa, and include most animal species such as mollusks, annelids, arthropods, and many nematodes.

Embryology and body plan are hallmarks of protostomes. They typically exhibit spiral, determinate cleavage of the early embryo, and many form the coelom by schizocoely, in which the mesoderm splits to create the body cavity. The blastopore usually becomes the mouth, with the anus forming later. There are exceptions and variations among lineages, including differences in coelom formation and larval development.

Larval forms and features vary across protostomes. Several groups have free-swimming trochophore-like larvae, especially within the trochozoans. Lophophorates possess a distinctive feeding structure called a lophophore. Ecdysozoans, including arthropods and nematodes, are characterized by periodic molting of their outer cuticle. Some ecdysozoans possess a pseudocoelom rather than a true coelom, as seen in nematodes, while many protostomes have true coeloms.

Examples and significance: The protostome lineage includes Mollusca, Annelida, Arthropoda, Nemertea, Platyhelminthes, and Nematoda, among others. Protostomes are a major component of animal biodiversity, and their developmental patterns have been central to studies of animal evolution, particularly in relation to deuterostome development. Molecular data support Protostomia as a coherent monophyletic group.