Compartments in Scyphozoa

Polyps of Scyphozoa have a cup-shaped body. At one end is the mouth opening surrounded by tentacles, at the other end is an attachment disc. The body wall consists of two tissue layers, the ectoderm and the endoderm, which are separated by an extracellular matrix, the mesoglea. The polyp's gastric cavity is subdivided by septa running from the apical end to the basal body end. The septa consist of two layers of endoderm and according to biology textbooks the number of septa is four. However, in rare circumstances Aurelia produces polyps with zero, two, six, or eight septa. We found that the number was always even. Therefore we propose that two types of endoderm exist, forming alternating stripes running from the oral body end to the aboral end. The stripes have some properties of developmental compartments. Where cells of different compartments meet, they form a septum. We also propose that the ectoderm is subdivided into compartments. The borders of the ectodermal and endodermal compartments are perpendicular to each other. Tentacles of the polyp and rhopalia (sense organs) of the ephyra (young medusa), respectively, develop at the border between two ectodermal compartments. The number can be even or odd. Rhopalia formation is particularly favored where two ectodermal and two endodermal compartments meet.