906 
THE VOYAGE OF H.M.S. CHALLENGER. 
this number is now much increased by the discovery of many new species and genera. 
Modifications of certain organs, perfectly novel, and of the greatest interest were found, 
but the most important results of this voyage are, that the general character of the 
abyssal fish-fauna, the abundance of fishes, and the exact depths to which they may 
descend, have been ascertained. The depths at which the fishes collected by the 
Challenger were taken cannot be received without some critical examination of each 
individual species ; seeing that no precaution was taken to keep the mouth of the dredge 
or trawl closed during its descent or ascent, the fishes may have been captured in various 
strata between the surface and bottom. The Naturalists have simply recorded the 
greatest depth reached by the dredge or trawl on each occasion, therefore before 
anything like a division into bathymetrical zones can be attempted, the observations 
of the Challenger Expedition must be confirmed and supplemented by other series of 
similar systematic observations. One of the most startling conclusions that would have 
to be drawn from the Challenger observations, if the greatest depths be taken as those 
at which the fishes lived, is, that some of the species of deep-sea fishes would range from 
a depth of some 300 fathoms down to one of 2000 fathoms, or, in other words, that a 
fish which has once attained that modification of its organisation which will enable it to 
exist under the pressure of half a ton, can easily accommodate itself to a pressure of two 
tons or more, a conclusion which requires to be confirmed by other observations. 
The greatest depth reached hitherto by a dredge in which fishes were collected is 
2900 fathoms. But the specimens thus obtained belong to a species ( Gonostoma 
microdon ) which seems to be extremely abundant in the upper strata of the Atlantic and 
Pacific, and were therefore most likely caught by the dredge in its ascent. The next 
greatest depth, viz., 2750 fathoms, must be accepted as one at which fishes undoubtedly 
do live ; the fish obtained from this depth in the Atlantic, Bathyophi s ferox, showing by 
its whole habit that it is a form living on the bottom of the ocean. 
The fish fauna of the deep sea is composed chiefly of forms or modifications of forms 
which are found represented at the surface in the cold and temperate zones, or which 
appear as nocturnal pelagic forms. 
The Chondropterygians are few in number, not descending to a depth of more than 
600 fathoms. The Acanthopterygians, which form a majority of the coast and surface 
faunas, are also scantily represented ; genera identical with surface types are confined to 
the same inconsiderable depths as the Chondropterygians, whilst those Acanthopterygians 
which are so specialised for the life in the deep sea as to deserve generic separation, range 
from 200 to 2400 fathoms. Three distinct families of Acanthopterygians belong to 
the deep-sea fauna, viz., Tracliypteridse, Lophotidae, and Notacanthidae. They consist 
respectively of three, one, and two genera only. 
Gadidse, Ophidiidoe, and Macruridse are very numerous, ranging through all depths; 
they constitute about one-fourth of the whole deep-sea fauna. 
