MABINE ARTHEOPODS 285 



decomposition. As long as the crabs are active and remain within 

 their shells you may assume that the conditions are favourable ; 

 but when they become sluggish in their movements, and leave 

 their homes, the sanitary condition of the aquarium should be 

 regarded with suspicion ; for hermit crabs, like many of the 

 marine tube worms, generally quit their homes when the condi- 

 tions are unfavourable, as if they preferred to die outside. 



The Common Hermit Crab (Pagurus Bernhardus), also known 

 as the Soldier Crab, on account of its very pugnacious habits, is 

 common almost everywhere on our coasts, and may be distin- 

 guished by the numerous little tubercles on the claws and on the 

 upper edge of the front legs ; and there are several other species, 

 belonging to the same genus, distributed more or less locally on 

 the various shores. All are similar in general structure and habits, 

 the various species being identified principally by means of their 

 colour, the variations in the form of the appendages, and the 

 general character smooth, tubercular, spiny, &c. of the exo- 

 skeleton. One species, found in the sandy bays of Cornwall, 

 burrows rapidly in the sand. 



Coming now to the true crabs the Brachyura, or Short-tailed 

 crustaceans, as sometimes distinguished from the Anomura we 

 find quite a variety of interesting creatures, many species of which 

 are always within the reach of the collector at work between the 

 tide-marks. In all these the abdomen is only slightly developed, 

 and is never used in swimming, being permanently folded beneath 

 the thorax. This portion of the body, however, is usually very 

 distinctly segmented, and if it be lifted from its position it will 

 be found that some of the segments bear appendages corresponding 

 with the swimmerets of the lobster. It is also wider in the female 

 than in the male, and crabs of the former sex may often be found 

 during the summer with the abdomen more or less depressed, and 

 the space beneath it quite filled with eggs. 



The upper surface of the carapace of crabs is often very 

 distinctly grooved, and it is interesting to note that these features 

 of the exo-skeleton are not merely of external significance, for they 

 usually correspond in position with various internal structures, 

 some of them denoting the areas of the insertions of important 

 muscles, and others enclosing the regions of certain of the internal 

 organs. 



It will be noticed, too, that the carapace, which in lobsters is 

 often less than half the length of the body, covers the entire body 



