Ball proposed that the presence of this glands was a synapomorphy of Dugesia and Neppia. The absence of these glands in some Neppia species (N. jeanneli, N. montana and N. schubarti) is thought to be a secondary loss.
Ball suggested that the presence of these glands was a defining characteristic of both Dugesia and Neppia. However, certain Neppia species (N. jeanneli, N. montana, and N. schubarti) lack these glands, which is thought to be a result of evolutionary loss.