The defining characteristic of the deuterostome is the fact that the blastopore (the opening at the bottom of the forming gastrula) becomes the anus, whereas in protostomes the blastopore becomes the mouth. The deuterostome mouth develops at the opposite end of the embryo from the blastopore and a digestive tract develops in the middle connecting the two.
The name deuterostome means "mouth second", and refers to one important developmental feature unique to this group. To understand this feature, a little animal embryology is necessary. In the earliest stages of embryo development, when there are only a few cells and the embryo resembles a tiny globe of cells, a small pucker develops on one side of the embryo. This grows into a pocket, and allows some cells to migrate inside to form an additional layer of cells within the outer layer. At this stage, the embryo is known as a gastrula. In the Protostomia, which is the other major group of the Bilateria, the mouth develops from the edge of this pocket, where the inner and outer layer of cells meet; the anal opening develops later. In the Deuterostomia, the reverse is true; the pocket edge develops into the anus, and the mouth is formed later. Hence the byline at the top of this page: "Your mouth comes second."
Read more here:
http://www.ucmp.berkeley.edu/phyla/deuterostomia.html
http://en.wikipedia.org/wiki/Deuterostome
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