Fig. [127].—Cymonomus granulatus, × 1. A.1, A.2, 1st and 2nd antennae; E, eye-stalk; S, extra-orbital spine of carapace. (After Lankester.)
Cymonomus granulatus (Fig. [127]) is an abyssal form that has been dredged from the Mediterranean and North Atlantic, in which the eye-stalks are curiously tuberculated, and the ommatidia of the eye are entirely unpigmented and degenerate, though a few corneal facets are still recognisable. This species is replaced by C. quadratus in the Caribbean Sea and by C. normani on the East African coast, in which the alteration of the eye-stalks into thorny, beak-like projections becomes progressively marked, and all traces even of the corneal facets disappear. This remarkable genus was mentioned in the excursus on Crustacean eyes on p. 149.
Fig. [128].—Calappa granulata, from in front, × ½. C, Hand of chelipede; T, walking legs. (After Garstang.)
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.
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.