Why do articulate lamp shells lack an anus?

Articulate lamp shells, which are members of the phylum Brachiopoda, possess a digestive system that includes a mouth, a saclike stomach, and an intestine, but they lack an anus. Because they have this blind intestine, waste material cannot be eliminated through a posterior opening; instead, these animals must excrete waste back through the mouth.1

While some articulate brachiopods may rely partly on dissolved nutrients to supplement their diet, they primarily feed by using a lophophore to filter food particles from seawater. To manage their waste, some species utilize a fold in their shells to help separate incoming, food-bearing currents from outgoing, waste-bearing currents. In contrast, inarticulate brachiopods, such as Lingula, do possess an anus.21