Protostomes are a major classification within the animal kingdom, encompassing many creatures. They are bilateral animals. Along with deuterostomes, this classification helps scientists understand fundamental evolutionary relationships. Protostomes are distinguished by unique embryonic development, marking a significant evolutionary branching point.
Defining Characteristics of Protostomes
A defining characteristic involves the blastopore, the first opening in the developing embryo. In protostomes, this blastopore typically develops into the mouth. A second opening forms later to complete the digestive tract, serving as the anus.
Cell division, or cleavage, is another distinguishing feature. Their embryonic cells undergo spiral cleavage, where cells divide at oblique angles, resulting in a rotational, stacked arrangement. Protostomes also exhibit determinant cleavage, meaning each embryonic cell’s developmental fate is established very early. If an early cell is removed, the embryo may not develop completely because its path was already set.
Coelom formation, the development of the body cavity, also occurs via schizocoely. This happens when the coelom forms from the splitting of solid masses of mesoderm. These mesodermal blocks separate, creating the fluid-filled body cavity that houses internal organs.
Major Groups of Protostomes
Protostomes are broadly categorized into two major superphyla: Lophotrochozoa and Ecdysozoa. This division is based on molecular evidence and shared traits.
Lophotrochozoa includes a wide range of soft-bodied animals. Examples are mollusks (snails, clams, octopuses), annelids (earthworms, leeches), and flatworms. These animals often grow incrementally, adding to their body size over time.
Ecdysozoa consists of animals that grow by molting, or shedding, their tough outer cuticle or exoskeleton (ecdysis). This superphylum includes arthropods (insects, spiders, crustaceans) and nematodes (roundworms).
Contrasting Protostomes and Deuterostomes
The animal kingdom is divided into protostomes and deuterostomes based on embryonic development. This distinction represents a major evolutionary split.
The fate of the blastopore is a primary difference. In protostomes, the blastopore develops into the mouth, with the anus forming later. Conversely, in deuterostomes, the blastopore forms the anus, and the mouth develops as a secondary opening. This distinction is reflected in their names: “protostome” means “first mouth” and “deuterostome” means “second mouth.”
Cleavage patterns also differ between the two groups. Protostomes exhibit spiral and determinant cleavage, where cells divide at an oblique angle and their developmental fates are fixed early. Deuterostomes undergo radial and indeterminate cleavage, where the fate of early embryonic cells is not determined as early. This allows each cell to potentially develop into a complete organism if separated.
Coelom formation also varies. Protostomes form their coelom through schizocoely, involving the splitting of mesodermal tissue. Deuterostomes form their coelom via enterocoely, where the coelom arises from outpocketings of the archenteron, or primitive gut. Examples of deuterostomes include echinoderms (starfish, sea urchins) and chordates (vertebrates like humans).