CRUSTACEA. 55 



lower and upper portion ; the lower portion is the smaller, 

 and contains the true gills, while the roof or dorsal part 

 "is to be regarded with certainty as a lung."' In the 

 crabs examined, the lateral cavities always contained air, 

 and only sufficient water to keep the parts moist. This 

 small amount of water is necessary, as it is a well-known 

 fact that the breathing organs of all animals can only absorb 

 oxygen in sufficient quantities when the surface membranes 

 are kept moist. 



The "lungs" are richly supplied with blood-vessels. 

 The situation of these vessels, the direction of the flow of 

 blood through them, and the fact that this blood cannot 

 be regarded as arterial, all tend to prove that these crabs 

 breathe by means of their lungs, and not by their gills. 



••Should the gills in the gill-cavity of a land-crab," 

 remarks Semper, "become yet more reduced, and at last 

 entirely disappear, then would the simple gill-cavity be 

 exactly equal, physiologically, to a true lung, as it is in 

 the air-breathing snails, though it might still be called a 

 gill-cavity, because it would be such morphologically."* 



The highest members of the Anomoura group, 

 Lithodes, are so like some of the true crabs, especially 

 the spider-crabs (see p. 61, Fig. 29), that they were 

 formerly classified with these in the same family. They 

 have the same form and general structure, but with only 

 four pairs of functionally useful walking-legs, the fifth 

 pair being very small, and the fifth ring to which these 

 are attached being separated from the other thoracic 

 rings, as in the hermit-crab. The abdomen is bent 

 under the body as in the true crabs, and covered with 

 soft skin on the inside, where it is protected, and with 

 three rows of plates on the outside, where it is ex- 

 posed, the terga in the centre, and the two rows of 

 epimera on the sides. 



