vi CYCLOMETOPA RESPIRATORY MECHANISMS 189 



pointed out,the Megalopa shows Portunid characters,and the resem- 

 lilauce to the Oxystomata in the front of the carapace and in the 

 mouth may be secondary. The respiratory arrangement of this 

 Crab has already been mentioned in comparing its structure with 

 that of the Mole-crab Alljunea. The form of the antenna! tube ca n 

 lie gathered from the figure of the Megalopa stage (Fig. 125, p. 183). 

 It should be noted that when the Crab is buried in the sand 

 with only the tip of the antennal tube projecting, the water is 

 sucked down and enters the branchial cavities anteriorly, the 

 antennal tube being continued by a tube formed from the third 

 maxillipedes and the forehead ; the water is exhaled at the sides 

 of the branchial cavities beneath the branchiostegites. Thus in 

 Corystes the normal direction of the current is reversed, but when 

 the Crab is not buried, and is moving over the surface, it breathes 

 iu the usual manner, taking in the water at the sides of the 

 branchiostegites and exhaling it anteriorly by the tube. The 

 related Atelecydus, found like Corystes very commonly at 1'1\ - 

 mouth, uses two methods of breathing: when it is in the 

 surface-layers of sand it makes use of its antennal tube, which 

 is. however, much shorter than in Corystes; but when it burro\\s 

 deeper, where the antennal tube is no use, it folds its chelipede- 

 and also its other legs, which are densely covered with bristles, 

 so as to form a reservoir of pure water underneath it free from 

 sand, which it passes through the gill -chambers in the usual 

 manner (see Garstang, loc. cit. p. 186). 



The respiratory adaptations in L///H/. Intxtntu and their con- 

 vergence towards those of the Oxystomatous Mafntn have IK-CM 

 idn-ady touched upon (pp. 186, 187). 



In this connexion must be mentioned the interesting experi- 

 ments of AN'. F. E. Weldon 1 upon the respiratory function- of 

 (' /-fin us maenfis at Plymouth, since these were the iirst note- 

 worthy observations directed towards the exact measurement of 

 tin- action of natural selection upon any animal, a Held of 

 observation in which AVeldou will always be looked upon a- a 

 pioneer. An extended series of measurements by AYeldm and 

 Thompson on male specimens of Carcinus nun- no* of vario 

 sizes bet \\een the years 1893 and IS'.'S slm\\ed a steady decrease 

 in the ratio o( carapace breadth to len-th; the ('nibs appealed 

 to be becoming steadily narrower across the frontal man: in, and 



1 /;,*. I:,', i. As*, for IV'-. > 



