_Corystes cassivelaunus_ is a Crab of doubtful affinities. It is sometimes placed among the Oxyrhyncha, but, as Gurney[154] has pointed out, the Megalopa shows Portunid characters, and the resemblance to the Oxystomata in the front of the carapace and in the mouth may be secondary. The respiratory arrangement of this Crab has already been mentioned in comparing its structure with that of the Mole-crab _Albunea_. The form of the antennal tube can be gathered from the figure of the Megalopa stage (Fig. 125, p. 183). It should be noted that when the Crab is buried in the sand with only the tip of the antennal tube projecting, the water is sucked down and enters the branchial cavities anteriorly, the antennal tube being continued by a tube formed from the third maxillipedes and the forehead; the water is exhaled at the sides of the branchial cavities beneath the branchiostegites. Thus in _Corystes_ the normal direction of the current is reversed, but when the Crab is not buried, and is moving over the surface, it breathes in the usual manner, taking in the water at the sides of the branchiostegites and exhaling it anteriorly by the tube. The related _Atelecyclus_, found like _Corystes_ very commonly at Plymouth, uses two methods of breathing: when it is in the surface-layers of sand it makes use of its antennal tube, which is, however, much shorter than in _Corystes_; but when it burrows deeper, where the antennal tube is no use, it folds its chelipedes and also its other legs, which are densely covered with bristles, so as to form a reservoir of pure water underneath it free from sand, which it passes through the gill-chambers in the usual manner (see Garstang, _loc. cit._ p. 186). The respiratory adaptations in _Lupa hastata_ and their convergence towards those of the Oxystomatous _Matuta_ have been already touched upon (pp. 186, 187).