In the next stage (“Furcilia”) the other abdominal pleopods are added, the whole series being completed before the thoracic appendages number more than two or three. This stage corresponds to the Metazoaea of the Decapoda, and the interference in the orderly differentiation of the segments with their appendages from before backwards is a phenomenon which we shall meet again when we treat of Decapod metamorphosis. It is evidently a secondary modification, furnishing the larva precociously with its most important swimming organs so as to enable it to lead a pelagic existence. The frequent violation of the law of metameric segmentation, that the most anterior segments being the first formed should be the first to be fully differentiated, leads us to suppose that the larval stages of the Eucarida at any rate do not represent phylogenetic adult stages through which the Malacostraca have passed. Nor do they, perhaps, even represent primitive larval stages, but have been secondarily acquired from an embryonic condition which used to be passed through within the egg-membranes, as in _Nebalia_ and the Mysidacea, when the order of differentiation of the segments was normal. The case is a little different with the Nauplius larva. This larval form, in an identical condition, is found both in the Entomostraca as a general rule, and again in certain Malacostraca, viz. the Euphausiidae and the Peneidea. Whatever its phylogenetic meaning may be, we may be quite certain that the ancestor of the two great divisions of the Crustacea had a free-swimming Nauplius larva, and this conclusion is confirmed by the probable presence of a Nauplius larva in Trilobites. The Euphausiidae, in contradistinction to the Mysidae, are frequently met with in the surface-plankton. _Euphausia pellucida_ (Fig. 102) is of universal distribution, and is frequently taken at the surface as well as at considerable depths.