A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction betweenprotostomes and deuterostomes is based on the direction in which the mouth (stoma) develops in relation to the blastopore. Protostome derives from the Greek word protostoma meaning "first mouth"(πρώτος + στόμα) whereas Deuterostome's etymology is "second mouth" from the words second and mouth (δεύτερος + στόμα).
The major distinctions between deuterostomes and protostomes are found in embryonic development:
In protostome development, the first opening in development, the blastopore, becomes the animal's mouth.
In deuterostome development, the blastopore becomes the animal's anus.