METAZOA CRUSTACEA. 353 



a formidable animal. The small abdomen is bent under 

 the triangular cephalothorax so that it resembles that of 

 the true crab soon to be described. There are, however, 

 but four pairs of functionally useful walking-feet, the fifth 

 pair being reduced in size and fastened to the last uncon- 

 solidated segment of the cephalothorax, as in the hermit 

 crabs. Lithodes has given up the habit of swimming and 

 is found creeping along the bottom with sluggish motion. 1 



Birgus latro Linn., or the palm crab, is an animal of 

 great interest, since it has not only changed its habitat 

 from the sea to the land but has also succeeded in con- 

 verting a part of its gill chamber into an air-breathing 

 lung. PI. 864, fig. i, is a diagrammatic representation 

 of the lungs and the circulation in this crab. The heart 

 is in the center and the lung cavities on either side. The 

 anterior blood vessels are the veins, and the large poste- 

 rior vessel, the artery. Fig. 2 is a diagram of a vertical 

 section of the same, showing the branchia and the lung 

 cavity with the pulmonary villi or tufts on the inner sur- 

 face of the wall. The dark circular spots are blood ves- 

 sels which have been cut across and which connect with 

 the heart. 



This crab spends its adult life on the land far from 

 water, but it goes to the sea to deposit its eggs and the 

 young are free-swimming creatures which breathe the air 

 dissolved in water. 



BRACHYURA. 



The development of our common crab, Cancer irroratus 

 Say (PL 865; Nos. 866-868), illustrates the mode of devel- 

 opment of most crabs. The nauplius stage is passed 

 through in the egg, the crab hatching as a zoe'a (PI. 865, 

 fig. i ; the line indicates natural size ; see also figure on 



^Dana, Crustacea U. S. Explor. Exped., 1,^852, p. 428. 



