186 CRUSTACEA——EUCARIDA——DECAPODA CHAP. 
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 
Kast African coast, in which the alteration of the eye-stalks into 
thorny, beak-hke projections becomes progressively marked, and 
all traces even ofthe corneal facets disappear. This remarkable 
genus was mentioned in the excursus on Crustacean eyes on p. 149. 
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.' This animal has the 
chelipedes wonderfully modified in structure, and when it is 
reposing in the sand 
it holds them ap- 
posed 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 prolonga- 
tion 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 ' 
(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 
Fic. 128.—Calappa granulata, from in front, x 4. C, Hand 
of chelipede ; 7. walking legs. (After Garstang.) 
' Garstang, Quart. J. Micr. Sci. x1., 1897, p. 211, and Journ. Mar. Biol. Ass. iv., 
1895-97, p. 396. 
