The anus from Latin anus meaning "ring", "circle" is an opening at the opposite end of an animal's digestive tract from the mouth.
the mouth anus From to the
The "head" and "tail" ends were assumed to correspond to the swimming direction of the larva. All animals start out in development with one hole, the blastopore. The genes employed in the embryonic construction of the flatworm mouth are the same as those expressed for the protostome and deuterostome mouth, which suggests that the structures are equivalent homologous , and that the older ideas about protostome mouth formation were correct. Uses editors parameter.
Most kindergarteners can tell you that an animal eats with its mouth, not its butt. This accompanied or facilitated other important evolutionary developments: