i86 



CRUSTACEA EUCARIDA DECAPODA 



the eye are entirely uupigmented 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. 

 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 l;)oth 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 Calapim} This animal has the 

 chelipedes woiulerfully 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 hanJcsii ^ 

 (Fig. 129), a swimming and fossorial Crfib found in the Indo- 

 Pacitic. 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 



^ Garstang, Quart. J. Micr. Sci. xl., 1897, p. 211, and Journ. Mar. Biol. Ass. iv., 

 1895-97, p. 396. 



Fig. 128. — Calappa gramdata, from in front, x h- C, Hand 

 of clielipede ; T. walking legs. (After Garstang.) 



