180 THE DISTRIBUTION OF 
depths. Thus the members of the genus Macrurus, 
believed to be true abyssal, that is ground-living, forms, 
have apparently such eggs and larvae. The abyssal 
hermit crabs, also, have bathypelagic larvae, just as 
the littoral forms have larvae which form part of the 
surface plankton. 
(3) The abyssal region of the oceans is contrasted 
with both the other regions in that its waters must 
be permanently calm. It is in harmony with this 
calm that we find, as already stated, some deep-sea 
sponges which have no attachment to the substratum, 
upon which they seem to lie loosely. The pressure 
must be enormous, for the animals of the depths sup- 
port a weight of several miles of sea-water upon their 
bodies. Though there are no plants, oxygen seems to 
be abundant. But this is a consequence of the well- 
developed circulation of the great oceans, and is not 
a feature of enclosed seas. Thus the Black Sea has no 
animals in its depths, and the waters there are impreg- 
nated with sulphuretted hydrogen, and contain but 
little oxygen. In the Mediterranean the conditions are 
less extreme, but the deep-sea fauna is scanty. Light 
is, of course, entirely absent in the depths. The water 
is permanently cold. The basal food-supply must 
consist of the dead animals of the upper zone, which 
fall downwards through the water as they die. In 
consequence we find that many of the abyssal forms 
are mud-eaters. To what extent the floor of the 
ocean is peopled still remains a difficult question. 
As already noted, Dr. Hjort believes that genuine 
abyssal fish, in the sense of ground forms, are few, 
most of the so-called abyssal forms being bathypelagic 
and having been obtained in the process of hauling 
in the dredge. But there can be no doubt that many 
