In protostomes, the anus arises secondarily.Protostomes contains less phyla and species than Deuterostomes.All the remaining bilaterian phyla are grouped into protostomes or divided into two groups: protostomes and lophophorate.Protostomes are less evolved and complex in their body compositions than Deuterostomes.It is also important to note that many deuterostomes also have pharyngeal gill slits.
The nervous system of protostomes is solid, ventral nerve cord.It usually develops into mosederm and endoderm of a living organism. Archenteron is a rudimentary alimentary cavity formed during the early stages of embryo development. There is no archenteron development in protostomes.In protostomes, the gut is tunneled into embryo and forms anus.Protostomes are referred to as schizocoelomatesbecause the coelom is developed by splitting of the solid mass of the embryonic mesoderm.The blastopore in a protostome develops into a mouth.Bivalve mollusks (clams, oysters, mussels, scallops).Crustaceans (crabs, lobsters, crayfish, woodlice, bernacles).Insects (beetles, ants, flies, crickets, butterflies, flees, cicada, bees).The coelom of the organism develops through a process called schizocoely, in which masses from the mesoderm migrate and form the coelom. The role is chosen, and stays as such in the organism’s adult form. Additionally the cells roles (i.e this cell will be a neuron) are determined very early on in development. Protostomes exhibit spiral clevage in their cells, since each layer is offest slightly. This pore deepens, forming the gut and eventually the anus. In other words, during protostome development the first opening to appear in a blastopore becomes the mouth of the organism. Typically, the protostome condition is defined by a spiral and determinant cleavage in the early stages of embryo development, schizocoelous coelom formation (as the archenteron (embryonic gut) forms the coelom begins as splits within the solid mesodermal mass, and the formation of the blastopore (the original opening) into the mouth. Protostomes are any of a major group of animals defined by its embryonic development, in which the first opening in the embryo becomes the mouth.
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