THE FISH OF THE DEEP SEA 151 



capable in the great number of cases of rapid and 

 vigorous movement, the muscles of the trunk and 

 tail are usually thin, and the fascicles loosely connected 

 with one another. 



Deep-sea fish are not characterised by an absence 

 of the swimming bladder. This organ occurs just as 

 frequently and in the same families as in the shallow- 

 water fauna, but we do not know whether it possesses 

 any special peculiarities or not, as it is usually so 

 ruptured and destroyed by the change of pressure 

 it undergoes in being brought to the surface, that it 

 is impossible to make any thoroughly accurate in- 

 vestigation of its anatomy and relations. 



The extraordinary development of glands in the 

 skin which secrete mucus, and the presence in many 

 forms of very highly specialised organs for emitting 

 phosphorescent light, are characters of the deep-sea fish 

 fauna, to which I have referred in a previous chapter. 



As with the Tunicates, some of the Crustacea 

 and other groups, the fish of the abysmal zone show 

 curious modifications of the respiratory system. The 

 gill laminas of these animals are not only reduced 

 in number, but appear to be short and shrunken. 

 It is possible, of course, that during life they may 

 end in fine delicate points which are broken 



