CROSS-EXAMINING A CRAB 87 



nected with the third and fourth pairs of legs, but that 

 these had anything to do with the branchial cavity was 

 not suspected. These two pairs are pressed more closely 

 together than the rest. The opposed surfaces of their 

 basal joints, that is, the hinder surface in the third, and 

 the front surface in the fourth pair, are flat and smooth, 

 and their margins are clos jly fringed with long, sheeny, 

 peculiarly formed hairs. Milne-Edwards, who compares 

 them to articular surfaces, as their appearance warrants, 

 thinks that they serve to diminish the friction between 

 the two legs. On this supposition the question arises why 

 precisely in these crabs and only between these two pairs 

 of legs such a provision for diminishing friction is neces- 

 sary, not to mention that it leaves unexplained the singular 

 hairs, which must augment instead of diminishing friction. 

 While, then, I was bending to and fro in ever so many 

 directions the legs of a large Sand-crab, in order to see 

 in what movements of the animal friction occurred at the 

 place in question, and whether perhaps these were move- 

 ments often recurring and of special importance to it, I 

 observed, when I had stretched the legs far apart, a round 

 opening of considerable size between their bases, through 

 which air could easily be blown into the branchial cavity 

 or even a slender probe be introduced. The aperture 

 opens into the branchial cavity behind a conical tubercle, 

 which stands above the third foot at the place of a bran- 

 chia which is wanting in Ocypoda. It is laterally bounded 

 by ridges which rise above the articulation of the legs and 

 to which the lower edge of the carapace is applied. Also 

 outwardly it is overarched by these ridges with the ex- 

 ception of a narrow slit. Over this slit extends the cara- 

 pace, which just at this point projects further downwards 

 than elsewhere, and so a complete tube is formed. While 

 Grapsus always admits water to its branchiae only from in 

 front, in Ocypoda I saw it also streaming in through the 

 just described aperture.' 



For its details about one particular crustacean such a 

 passage is interesting, but it is far more important as a 

 lesson in scientific observation. There are numbers of 



