The Oxystomata, like the Cyclometopa, to be considered later, live in sandy and gravelly regions, and burrow to a greater or less extent, and we find in both groups admirable adaptations for securing a pure stream of water, uncontaminated by particles of sand, for flushing the gills. Perhaps the most remarkable of these adaptations is afforded by _Calappa_.[153] This animal has the chelipedes wonderfully modified in structure, and when it is reposing in the sand it holds them apposed to the front of the carapace, as shown in Fig. 128, so that the spines upon their edges, together with the hairy margin of the carapace, form a most efficient filter for straining off sand and grit from the stream of water which is sucked down between the closely-fitting chelipedes and carapace, to enter the branchial chambers at their sides. The exhaled current of water passes out anteriorly through a tube formed by a prolongation of the endopodites of the first maxillipedes. The exhalant aperture is shown in Fig. 128 by the two black cavities below the snout in the middle line. A similar method is pursued by the related _Matuta banksii_[153] (Fig. 129), a swimming and fossorial Crab found in the Indo-Pacific. In this Crab the chelipedes also fit against the carapace to form a strainer, and their function is assisted by the enlargement of the posterior spine, which acts as a kind of elbow-rest to keep the chelipedes properly in position. The inhalant openings are situated just in front of the chelipedes. It is a most remarkable fact that among the Cyclometopa, _Lupa hastata_ (Fig. 131) has an exactly similar arrangement. Apparently we have here another instance of convergence, similar to that of _Corystes_ and _Albunea_, but the case is complicated by the fact that some of the Oxystomata, and among them _Matuta_, show a certain amount of relationship to the Cyclometopous Portunids, so that it is just conceivable that the resemblances in the respiratory arrangement are due to a common descent and not to convergence. [Illustration: FIG. 129.—Dorsal view of _Matuta banksii_, × 1. (From an original drawing prepared for Professor Weldon.) ] In the Leucosiidae, of which the Mediterranean _Ilia nucleus_ (Fig. 130) is an example, the inhalant aperture is situated between the orbits, and leads into gutters excavated in the “pterygostomial plates” flanking the mouth, which are furnished with filtering hairs and are converted into closed canals by expansions of the exopodites of the third maxillipedes. Thus these Crabs possess a filtering apparatus independent of the chelipedes and of the margin of the carapace.