From Academic Kids
Deuterostomes (taxonomic term: Deuterostomia; from the Greek: "other mouth") are a taxon of animals. They are a subgroup of the Bilateria, and are opposed to the protostomes. Deuterostomes are distinguished by their embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes it becomes the mouth.
There are three phyla of deuterostomes:
- Phylum Chordata (vertebrates and their kin)
- Phylum Echinodermata (starfishes, sea urchins, sea cucumbers, etc.)
- Phylum Hemichordata (acorn worms)
In both deuterostomes and protostomes, a zygote first develops into a hollow ball of cells, called a blastula. In deuterostomes, the early divisions occur parallel or perpendicular to the polar axis. This is called radial cleavage, and also occurs in certain protostomes, such as the lophophorates. Cleavage is indeterminate - the cells' fates are not determined early on. Thus if the first four cells are separated, each cell is capable of forming a complete small larva, and if a cell is removed from the blastula the other cells will compensate.