CRUSTACEA OF THE SEASHORE 101 



above. Since this tube communicates at its base 

 with the passages through which the water passes 

 out from the gill chamber in most Crabs, it was 

 assumed by the older observers that the antennal 

 tube served to carry the outflowing water to the 

 surface of the sand. It has recently been shown, 

 however, by Professor Garstang that when the 

 Masked Crab is buried in sand the normal respiratory 

 current is reversed, water being drawn down the 

 antennal tube, into the gill chambers, and passing 

 out through the openings at the base of the chelipeds 

 which, when the Crab is not buried, serve for its 

 entrance. 



Most, if not all, of the Crabs belonging to the tribe 

 Oxystomata are sand-burrowers, and the structure of 

 the mouth parts characteristic of the tribe appears 

 to have been acquired as an adaptation to this habit. 

 As already mentioned, the mouth-frame in these 

 Crabs is triangular instead of square, being produced 

 forwards between the eyes, and the third maxillipeds, 

 which cover it, are also elongated. In this way the" 

 exhalent channels carrying the water from the gill 

 chambers open on the front margin of the head, and 

 are exposed even when the Crab is buried. In the 

 different families of this tribe the inhalent openings 

 by which the water enters the gill cavities are pro- 

 tected in various ways, and so arranged that respira- 

 tion can go on without danger of the gills becoming 

 clogged by sediment. 



