REPORT ON THE DEEP-SEA FISHES. XXV 



feeble, so that the specimens require the most careful handling to prevent their breaking 

 up into fragments. This condition appears to obtain in the highest degree in a fish 

 allied to Plagyodus, and occurring in the Madeiran Sea, of which Lowe succeeded in 

 obtaining fragments only. Specimens of this fish are sometimes caught on long lines at 

 great depths, but before they can be hauled to the surface, the body breaks away, leaving 

 fragments only on the hook.^ We cannot assume that this loose connection of the osseous 

 and muscular systems obtains whilst those fishes remain under the normal physical 

 conditions of their abyssal abode. All are carnivorous, and some of them 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 strong, and the chain of the segments of their vertebral column as firmly 

 linked together, as in surface fishes. In coming to the surface their body has undergone 

 a change which is merely due to their rapid withdrawal from the pressure under which 

 they lived ; it is a much aggravated form of the afiection that is experienced by persons 

 reaching great altitudes in a balloon, or by the ascent of a mountain. In every living 

 organism with an intestinal tract there are accumulations 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 expand, so that, if the with- 

 drawal from a depth is not an extremely slow and gradual process, the various tissues 

 must be distended, loosened and ruptured ; and what is a vigorous fish at a depth of 500 

 or more fathoms, appears at the surface as a loosely-jointed body, which, if the skin is not 

 of suflicient toughness, can only be kept together with difliculty. At great depths a 

 fibrous osseous structure and a thin layer of muscles suffice to obtain the same results for 

 which, at the surface, thickness of muscle and firm osseous tissue are necessary. 



The singular circumstance that the first two specimens known of Saccopteryx, two of 

 Ghiasmodus, and one of Omosudis were picked up floating on the surface, dead or in a 

 dying condition, with their stomach distended by a large recently swallowed prey, can be 

 easily explained by the aid of those phj^siological facts. If, during the struggle which 

 must take place between the attacking fish and its prey, the fishes are carried out of their 

 depth into a somewhat higher stratum, the expansion of the gases will cause both to 

 ascend towards the surface, especially if one of them be provided with an air-bladder, and 

 the rate of speed of the ascent will increase the nearer they approach the surface, which 

 they reach dead or in a dying condition, as witnessed and described by Lowe {vide supra, 

 p. XX.). Occurrences of this kind must happen very often, as, of course, comparatively 

 few can by accident fall under the observation of naturalists. 



In a slight degree the phenomenon described is a matter of every day occurrence, 

 well known to fishermen who fish at depths of from 40 to 80 fathoms. The fishes with- 

 drawn from that depth come up more or less distended, if they possess a closed air-bladder. 



1 IViegm. Arch., 1860, p. 123. 



(ZOOL. CHALL. EXP. PART LVII. — 1887.) " I^H '^ 



