The members of this sub-order, although agreeing with the Isopoda in the essentials of their anatomy, resemble the Amphipoda in being rather laterally compressed, and in having the hand of the first free thoracic limb enlarged and subchelate. The abdomen is greatly produced laterally by expansions of the segments. In fact, the shape of the body and of the limbs is very Amphipodan.—_Phreatoicus_ from New Zealand, Southern Australia, and Tasmania. _Phreatoicopsis_,[111] a very large form from Gippsland, Victoria. Only one family exists, =Phreatoicidae=. =Order IV. Amphipoda.= In this order the body is flattened laterally, the heart is anterior in position, and the branchial organs are attached to the thoracic limbs. There are three well defined sub-orders, (i.) the Crevettina, including a vast assemblage of very similar animals, of which the common _Gammarus_ and _Orchestia_ may serve as examples; (ii.) the Laemodipoda or Caprellids, and (iii.) the Hyperina. We cannot do more than touch on the organisation of these sub-orders. =Sub-Order 1. Crevettina.= In this sub-order only one thoracic segment is fused with the head; the basal joints of the thoracic limbs are expanded to form broad lateral plates, and the abdomen is well developed, with six pairs of pleopods, the last three pairs being always turned backwards, and stiffened to act as uropods. This group has numerous fresh-water representatives, _e.g._ _Gammarus_ of several species, the blind well-shrimp _Niphargus_, and the S. American _Hyalella_; but the vast majority of the species are marine, and are found especially in the littoral zone wherever the rocks are covered with a rich growth of algae, Polyzoa, etc. The Talitridae or “Sand-hoppers” have deserted the waters and live entirely in the sand and under rocks on the shore, and one common European species, _Orchestia gammarellus_, penetrates far inland, and may be found in gardens where the soil is moist many miles from the sea. The Rev. T. R. R. Stebbing, in his standard work[112] on this group, recognises forty-one families, and more than 1000 species, so that we can only mention a few of the families, many of which, indeed, differ from one another in small characters. =Fam. Lysianassidae.=—The first joint of the first antenna is short, with an accessory flagellum. Mandible with a palp, and with an almost smooth cutting edge. The third joint of the second gnathopod is elongated. This family is entirely marine, comprising forty-eight genera, with species distributed in all seas. One genus, _Pseudalibrotus_, inhabits the brackish water of the Caspian Sea. _Lysianassa_ has several common British and Mediterranean species.