A mature zooid has three important regions, the preoral disc or cephalic shield, the collar and the trunk. In the collar, the mouth and anus (U-shaped digestive system) and arms are found; Graptholitina has a single pair of arms with several paired tentacles. As a nervous system, graptolites have a simple layer of fibers between the epidermis and the basal lamina, also have a "collar ganglion" that gives rise to several nerve branches, similar to the neural tube of chordates. All this information was inferred by the extant "Rhabdopleura", however, it is very likely that fossil zooids had the same morphology. An important feature in the tubarium is the fusellum, which looks like lines of growth along the tube observed as semicircular rings in a zig-zag pattern.