Evolution & Systematics

The phylogeny and systematics of the Hydrozoa are still highly debated (Schuchert P. 2011). Looking at it from an evolutionary point of view and starting at the phylum Cnidaria (Ruppert 2004). What separates the subphylum Medusozoa from the other Cnidaria is the medusa (non-polyp) stage in their life-cycle known as jellyfish. The Hydrozoa have bulbs at the bases of their tentacles, they have a medusa with a velum and the nematocysts are restricted to the epidermis. The hydrozoa are then split into A-Forms and L-Forms. The A-Forms have manubrial gonads, they are athecate (periderm does not enclose the hydranth). In the L-Forms the medusa have statocysts, they are thecate and the gonads are found on the radial canals.