with compound stalked eyes and have a carapace which covers most of the thorax but does not fuse with the last four thoracic segments (as instead is seen in the Eucarida), they possess well-developed thoracopods (for swimming) and tail fan, they also have similar behaviour (swarming) and foregut structure; Pygocephalomorph, a fossil from the Permian, appears similar to the Lophogastrida, which in addition to other factors, has traditionally allowed the Lophogastrida to be identified as the more primitive Mysidacea. However, the monophyly of the Mysidacea been disputed, due to molecular data and several differences, one profound difference between the two taxa is that in the Mysida the carapace acts as a respiratory surface due to the absence of gills, which are however present in the Lophogastrida. When they are considered together many authors have put Mysidacea basal to the remaining peracarids ((paraphyletic)Ruppert & Barnes, 1994; (monophyletic)), others have even either grouped them with Euphausiacea to form the Schizopoda or made them basal to a eucarid subtree (paraphyletic). The Isopoda and the Amphipoda are two of the largest pericarid groups, they both lack a carapace, possess sessile compound eyes and lack a sharp demarcation between thoracic and abdominal segments, but however differ in several features, such as gills. Isopoda contains 10,000 species, the organisms are dorsoventrally flattened and occupy not only marine and freshwater habitats, but even terrestrial (woodlice) for which they developed a thickened cuticle and gas exchange organs, allowing them to live even in arid regions. Amphipoda is a highly diverse group of 8,000 species, ranging from the Caprellida with a long and narrow body shape (skeleton shrimp) to the shrimp-like Gammaridea (scuds and sand hoppers). The position of the Isopoda and the Amphipoda amongst the Peracarida is also debated, some authors support a derived united group (Edriophthalma) which is either monophyletic or paraphyletic (Wheeler, 1998), others support a basal paraphyletic Isopoda and Amphipoda group (Watling, 1999); however, other authors believe that several features that unite the Isopoda and the Amphipoda are homoplasious and that the two groups reside with different groups: one proposed a basal Amphipoda to a clade formed from Isopoda + Tanaidacea and Cumacea + Mictacea + Speleogriphacea, while some older phylogenetic trees place the Amphipoda and the Mysidacea basal to the Peracarida (without the Thermosbaenacea) either as a polyphyletic or a monophyletic group, whereas the Isopoda are in a derived clade with Tanaidacea. Molecular studies