THE DECAPOD CRUSTACEA. 191 



The masked crab is very easily recognised owing to the 

 peculiarly elongated shape of the carapace, which terminates 

 anteriorly in a deeply notched rostrum, and is furnished 

 with three distinct spines. These points are readily made 

 out in the figure. The elongated antenme are also peculiar 

 and characteristic. As already indicated, the nature of 

 their insertion is such that the inner surfaces of their flagella 

 are, or can be, closely apposed. These apposed surfaces are 

 densely fringed with hair, and, according to Mr. Garstang, 

 the respiratory current is at least at times downward through 

 the tube formed by the antennae. It will be recollected 

 that in Crustacea in general the gills are washed by a 

 constant stream of water which enters the gill-chamber at or 

 near its posterior end, and leaves it anteriorly near the 

 mouth. We have already noticed in the mud-loving Por- 

 cellana platycheles that the numerous hairs covering the 

 body sift the mud from the incoming water, and so protect 

 the delicate gills from injury. The masked crab usually 

 lives buried in sand, with only its long feelers protruding. 

 It is obvious that in this position it is almost impossible 

 that the respiratory current should be of the usual postero- 

 anterior type, and we therefore find that it is at least at 

 times reversed, entering at the anterior end of the gill- 

 chamber after passing down the antennal tube, and leaving 

 at its posterior end. The dense hairy fringe of the antenna 

 sifts out the particles of sand just as the mud is sifted out 

 in Porcellana. The great flexibility of the antennal stalks 

 also permits of the periodic cleaning of the antennas by the 

 drawing of one over the hairy surface of the other. 



It is interesting to note that another member of the 

 family Corystidee, Atelecyclus heterodon, also occurring on 

 British coasts, approaches both in appearance and in struc- 

 ture the crabs much more nearly than does Corystes itself 

 It is exclusively an inhabitant of deep water. 



