Reed (1916, p. 174) states that it may be either the representative of the "dorsal" organ of the branchiopods, or a median unpaired ocellus.
Ruedemann (1916) has made the only real investigation of the subject. He came to the conclusion that it was a parietal eye, without a crystalline lens, but corresponding to the "parietal eye of other crustaceans, and especially of the phyllopods, which is a lens-shaped or pear-shaped sac, usually filled with sea water." He found that above the "ocellus" the test was usually thin or even absent, and in a few cases a dark line beneath seemed to outline the original form of the sac. His summary follows:
It is claimed that most, if not all, trilobites possessed a median or parietal eye on the glabella. [In proof of this assertion the following facts are stated:]
1. A great number of species, belonging to more than thirty genera, possess a distinct tubercle on the glabella. This tubercle occurs alone in many genera, otherwise smooth, as in the Asaphidæ, and is hence of functional importance.
2. In certain cases, as in Cryptolithus tessellatus, distinct lenticular bodies [not lenses] were recognized; in others, as in Asaphus expansus, only a thinner, probably transparent test. Many other species show a distinct pit in interior casts of the tubercle, indicating a lens-like thickening of the top of the tubercle. The median eye therefore probably possessed all the different stages of development seen in other crustaceans.
3. As in the parietal eyes of the crustaceans and the eurypterids, the tubercles are most prominent and distinct in the earlier growth-stages, notably so in Isotelus gigas.
4. The tubercle is especially well developed in the so called blind forms where the lateral eyes are abortive, as in Cryptolithus (Trinucleus), Dionide, Ampyx.
5. The tubercles always appear on the apex on the highest part of the glabella, where their visual function would be most useful.
6. The tubercle is generally situated between the lateral eyes, like the parietal eye in crustaceans and eurypterids, on account of its close connection with the brain.
7. Frequently it forms the posterior termination of a short crest, also as in certain eurypterids (Stylonurus), indicating the direction of the nerve.