DEEP-SEA FISHES. 299 



having any of the characteristics of the dwellers of the deep 

 sea, must be regarded as surface-fishes. 



The most striking characteristic, found in many Deep-sea 

 fishes, is in relation to the tremendous pressure under which 

 they live. Their osseous and muscular systems are, as com- 

 pared with the same parts of surface-fishes, very feebly de- 

 veloped. The bones have a fibrous, fissured, and cavernous 

 texture; are light, with scarcely any calcareous matter, so 

 that the point of a needle will readily penetrate them with- 

 out breaking. The bones, especially the vertebrae, appear to 

 be most loosely connected with one another ; and it requires 

 the most careful handling to prevent the breaking of the 

 connective ligaments. The muscles, especially the great 

 lateral muscles of the trunk and tail, are thin, the fascicles 

 being readily separated from one another or torn, the connec- 

 tive tissue being extremely loose, feeble, or apparently absent. 

 This peculiarity has been observed in the Trachypteridm, 

 Plagyodus, GMasmodtis, Melanocetus, SaccopharyTws. But we 

 cannot assume that it actually obtains whilst those fishes 

 exist under their natural conditions. Some of them are most 

 rapacious creatures which must be able to execute rapid and 

 powerful movements to catch and overpower their prey ; and 

 for that object their muscular system, thin as its layers may 

 be, must be as firm, and the chain of the segments of their 

 vertebral column as firmly linked together as in surface-fishes. 

 Therefore, it is evident that the change which the body of 

 those fishes has undergone on their withdrawal from the pres- 

 sure under which they live is a much aggravated form of the 

 affection that is experienced by persons reaching great alti- 

 tudes in their ascent of a mountain or in a balloon. In every 

 living organism with an intestinal tract there are accumula- 

 tions of free gases; and, moreover, the blood and other 

 fluids, which permeate every part of the body, contain gases 

 in solution. Under greatly diminished pressure these gases 



