176 



CRUSTACEA EUCARIDA DECAPODA 



CHAP. 



segment; following these there are four large plates (1-4) 

 representing the terga of the first four abdominal segments ; the 

 fifth, sixth, and the telson are, as has been stated, carried on the 



under side of the abdo- 

 men, but they are re- 

 presented diagrammati- 

 cally (5, 6, T) in the 

 dorsal view. Besides 

 the large terga, there 

 are a number of small 

 plates laterally, usually 

 two to each segment, 

 but they show a ten- 

 dency to subdivide and 

 increase in the largest 

 specimens. This condi- 



FIG. 120. Dorsal view of abdomen, A, of Oenobita, tion of affairs is very 

 sp.; B, ..f Birguslatro. T, Telson; 1-6, lst-6th different to that in the 

 abdominal segments. 



naked fleshy abdomen 



of an ordinary Pagurid, but it can easily be deduced from 

 that of the genus Cenolita, ordinary Hermit-crabs found in the 

 Indo-Pacific Oceans, from which the Robber-crab has evidently 

 descended. In Cenobita (Fig. 120, A) we see the same system 

 of plates u} >on the dorsal surface of the abdomen, but they are 

 much smaller, and the lateral plates are not so numerous ; 

 indeed, the greater part of the abdomen remains fleshy and 

 imcalcified. The under surface of the abdomen shows the same 

 rugosity as is found in Birijus, and from a number of other 

 anatomical characters it is evident that the Eobber-crab is a 

 highly modified Cenobita that has deserted its shell and developed 

 a symmetrical abdomen protected by expanded and hardened 

 plates which represent those found in a reduced condition in 

 Cenolita. The species of Cenolita although they inhabit shells 

 ,iiid have normal branchiae, live on the shore, and have not been 

 seen to descend actually into the sea. 



The Lithodidae, which are found in temperate seas, especially 

 on the Northern Pacific coasts (though Litliodes maia occurs in 

 the North Sea, and certain species inhabit deep water in the 

 Indian Ocean), have a deceptively Brachyuran appearance, the 

 thorax being much shortened and the abdomen being much 



