244 THE ANTARCTIC MANUAL. 
by means of which a moving animal determines its position in rela- 
tion to the surroundings is as great at two thousand fathoms as at 
twenty, and, as far as is known, these organs have, as a whole, 
suffered no degeneration. 
If we could see the bottom of the deep sea, we should see, except 
in those few places where a current is active, a uniform layer of what 
we should call fine mud lying like a thick deposit over the bottom of 
the ocean, covering rock and stone. This is caused by the tireless 
fall of the skeletons of minute organisms which abound in the surface 
of the sea. In comparatively shallow waters, where the calcareous 
shell can reach the bottom without being dissolved in transit, this 
deposit is the Globigerina ooze; in deeper waters the siliceous frame- 
work of Radiolaria or Diatoms forms the deposit. It is ceaselessly, 
though very slowly and very gently, falling through the water. It 
is, perhaps, connected with this layer of deposit that so many mem- 
bers of the Benthos,* whose allies are sessile in shallower seas, are 
stalked, or if their allies be stalked, in the depths they have acquired 
longer stalks. Instances of this occur in the Sponges, the Alcyonarian 
Umbellula, the stalked Crinoids, the Tunicata, and other groups. 
Certain curious features occur over and over again in deep-sea 
creatures for which there seems no obvious reason. There is an in+ 
explicable inability to deposit much calcareous matter in the skeleton, 
whether internal or external. The bones of many deep-sea fishes are 
deficient in lime, and remain either membranous and fibrous or car- 
tilaginous; the shells of certain molluscs are “as tissue paper ;” 
the test of some Echinoids is soft, with at most certain detached cal- 
careous plates ; the exoskeletons of the Crustacea remain, as a rule, 
chitinous; calcareous sponges are not found. This state of things 
cannot be due to the absence of calcium carbonate, for we know that 
the calcium sulphate, from which the carbonate is formed, is present 
in abundance, and, moreover, the deficiency in lime occurs as com- 
monly in those creatures living on the calcareous Globigerina ooze as 
it does in those which live in the siliceous Radiolarian deposits. On 
the other hand many animals such as the Radiolaria which secrete 
silex have unusually stoutly developed skeletons at great depths. 
Possibly, connected with this deposit of sediment is the fact, that 
in many of the creatures who have retained their eyes these organs 
are borne at the end of long stalks. The Podophthalmous Crustacea 
tend to become more Podophthalmous, and some of the bathybial 
fishes carry their eyes at the tips of long lateral prolongations of the 
* A term introduced by Haeckel to denote those marine animals which do not swim 
about or float, and which live on the bottom of the ocean either sessile or creeping about. 
