Protostomes and deuterostomes represent the two largest evolutionary superphyla within the Animal Kingdom’s Bilateria. This fundamental division organizes the vast majority of animal life and is based on profound differences that occur during the initial stages of embryonic development. Understanding these distinct early processes is foundational to classifying and tracing the evolutionary history of complex animal forms.
The Primary Distinction: Fate of the Blastopore
The most direct distinction between these two superphyla comes from the literal meaning of their names, which describe the fate of the first opening of the developing gut. Protostome means “first mouth,” while Deuterostome translates to “second mouth.” This distinction centers on gastrulation, a phase where the single-layered blastula reorganizes into the multi-layered gastrula.
During gastrulation, an indentation forms on the embryo, creating the blastopore, which is the initial opening into the primitive gut (archenteron). In Protostomes, this blastopore becomes the permanent mouth, establishing the anterior end of the digestive tract. The anus then forms secondarily at the opposite end of the embryo.
The pattern is reversed in Deuterostomes, where the blastopore develops into the anus. The digestive tract extends toward the other pole of the embryo, and the mouth forms secondarily. This reversal—mouth-first versus anus-first—represents a deep divergence, defining how the entire body axis and gut tube are initially organized.
This difference determines the final layout of the animal’s digestive system, which is essentially a tube running from the mouth to the anus. This difference in development was historically the single most important characteristic used to separate these two massive groups of animals.
Early Embryonic Differences: Cleavage and Coelom Formation
Beyond the blastopore’s fate, two other major developmental processes—cleavage and coelom formation—follow distinct patterns. Cleavage refers to the rapid cell divisions that transform the fertilized egg (zygote) into a ball of cells called the blastula. The orientation of these early cell divisions differs significantly between the two groups.
Protostomes exhibit spiral cleavage, where the planes of cell division are oblique to the vertical axis of the embryo. This results in the upper cells resting in the furrows between the lower cells, creating a characteristic spiral arrangement. Protostomes also undergo determinate cleavage, meaning the developmental fate of each embryonic cell is established very early on.
If a cell (blastomere) is isolated from a four-cell Protostome embryo, it develops into only a partial organism because its developmental program is fixed. In contrast, Deuterostomes exhibit radial cleavage, where the cleavage planes are either parallel or perpendicular to the vertical axis. This results in the cells being stacked directly on top of each other, creating a symmetrical, radial pattern.
Deuterostome cleavage is indeterminate, meaning the cells produced during the early divisions retain the flexibility to develop into a complete organism. This regulative development means that separating the cells at the four-cell or eight-cell stage can result in multiple complete embryos. This is the mechanism allowing for the natural occurrence of identical twins in many Deuterostome species.
The formation of the coelom, the main body cavity that houses the internal organs, also follows two divergent paths. In Protostomes, the coelom forms through schizocoely, which translates to “split coelom.” This occurs when solid masses of mesoderm—the middle embryonic germ layer—split open internally to create the coelomic space.
Deuterostomes form their coelom through enterocoely, or “gut coelom.” During this process, the mesoderm arises from out-pocketings of the archenteron (the primitive gut). These pouches pinch off from the gut lining and expand to form the coelom, establishing the body cavity by evagination rather than internal splitting.
Classification and Examples
The developmental differences between Protostomes and Deuterostomes created two major evolutionary branches that diverged over 600 million years ago. Protostomes include the majority of animal species and are further divided into two major clades: the Ecdysozoa and the Lophotrochozoa.
Well-known examples of Protostomes include the phyla:
- Mollusca (snails, clams, and octopuses).
- Annelida (segmented worms like earthworms).
- Arthropoda (insects, spiders, and crustaceans).
The vast number of species in the Protostome group highlights its evolutionary success.
The Deuterostomes represent a smaller collection of animal phyla, primarily Echinodermata and Chordata. Echinoderms include organisms such as sea stars, sea urchins, and sea cucumbers, which exhibit five-part radial symmetry as adults despite their bilateral larval stage.
The phylum Chordata includes all vertebrates (fishes, amphibians, reptiles, birds, and mammals, including humans) as well as simpler organisms like tunicates. Humans are classified as Deuterostomes because our early embryonic development follows the anus-first, radial, and indeterminate pattern.