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 Al/bunea. The form of the antennal tube can 
be 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 
in the usual manner, taking in the water at the sides of the 
branchiostegites and exhaling it anteriorly by the tube. The 
related Atelecyclus, 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 J/atuta have been 
already touched upon (pp. 186, 187). 
_ In this connexion must be mentioned the interesting experi- 
ments of W. F. R. 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 1895 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. 
