Ascidians were first classified as Tunicata by Lamarck in 1816, although they were thought to belong to the Mollusca phylum (reviewed in Kott, 2005). The fact that tunicates belonged to the Chordata phylum was only discovered in 1867 by Kowalevski, when the study of their larvae demonstrated that ascidians embryos shared chordate characteristics (reviewed in Kott, 2005). Tunicata were divided into the three currently known classes by Herdman (1882): Ascidiacea, Thaliacea and Appendicularia. A. glabra belong to Ascidiacea class, which was divided into three orders by Lahille in 1886: Aplousobranchia, Phlebobranchia (Enterogona) and Stolidobranchia (Pleurogona) (reviewed in Shenkar et al., 2012). Due to their unpaired gonads located anteriorly to the gut loop, A. glabra were classified into the Enterogona (Phlebobranchia) Order (Klauss, 1978). The family Ascidiidae, which includes A. glabra, contains ascidians characterised by their firm, inflexible, translucent tunic, their flat branchial sac and the large gonads located next to the gut loop (Kott, 2005).