vi CYCLOMETOPA RESPIRATORY MECHANISMS 189 



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

 blance 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 Albunea. The form of the antennal tube can 

 be gathered from the figure of the Megalopa stage (Fig. 12 5, 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 

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

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

 in 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 Ply- 

 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 burrows 

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

 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 Lupa, hastata and their con- 

 vergence towards those of the Oxystomatous Matuta have been 

 already touched upon (pp. 186, 187). 



In this connexion must be mentioned the interesting experi- 

 ments of W. F. E. Weldon * upon the respiratory functions of 

 Carcinus maenas at Plymouth, since these were the first note- 

 worthy observations directed towards the exact measurement of 

 the action of natural selection upon any animal, a field of 

 observation in which Weldon will always be looked upon as a 

 pioneer. An extended series of measurements by Weldon and 

 Thompson on male specimens of Carcinus maenas of various 

 sizes between the years 1893 and 1898 showed a steady decrease 

 in the ratio of carapace breadth to length ; the Crabs appeared 

 to be becoming steadily narrower across the frontal margin, and 



1 Rep. Brit. Ass. for 1898, p. 887. 



