Because they grew by molting, sometimes only sections are found. The glabella is the only part of the cephalon that typically shows segmentation. The latter falls separately during molting. The position of the facial suture varies among taxonomic groups. It extends from the anterior margin past the inner side of the eyes and then can take one of at least four paths to the posterier edge of the cephalon.
Simple eyes consist of a single lens with a small transparent calcite node on the exoskeleton.
There are sensory and pigmented cells beneath the node. Each has an outer lens and an underlying sensory region. They appear smooth because the lenses are covered with a continuous layer of chitin that serves as a cornea. The holochroal eye is the more primitive form and is found from the Cambrian to the Permian. Schizochroal eyes are known only from the Ordovician to the Devonian.