716 
DE. WYVILLE THOMSON ON HOLTENIA 
part of the mud consists chiefly of an amorphous calcareous paste, produced by the 
disintegration of the Ehizopod shells, partly by pressure and the decomposition of the 
cementing animal matter, and partly by the solution in the sea-water of a portion of the 
carbonate of lime in the presence of disengaged carbonic acid ; while the surface-layer 
consists almost entirely of living Ehizopods, principally Globigerince. The sponges live 
buried in the upper layer of the mud. They are frequently so coloured and saturated 
with the chalk-mud up to a certain point, that one can tell at once to what extent they 
have been immersed. From the low density of the diffused protoplasm of the living 
forms, the condition of fine division of the carbonate of lime, the slightly increased 
density of the water, and the stillness, the transition from a semifluid to a gradually 
solidifying mud must be very gradual. There are two ways in which the bodies of the 
sponges may support themselves and maintain their position in this treacherous medium, 
and both seem to be adopted by different genera. Holtenia stretches out an unlimited 
maze of light spreading fibres, thus increasing its surface indefinitely without adding 
greatly to its weight. Hyalonema sends down perpendicularly a coil of strong spicules 
which gradually spread till they reach a stratum firm enough to give them anchorage. 
The question naturally arises, how is this enormously extended layer of animal life 
nourished 1 The dredge brings up no plants from great depths ; and although from the 
fact that many of the higher invertebrates which are associated with the sponges and 
rhizopods have eyes and are brightly coloured, it seems probable that a certain amount 
of light penetrates to the abysses ; it certainly does not appear to be in the form which 
is necessary for the decomposition of binary compounds by plants. To meet this diffi- 
culty it has been suggested that the Protozoa, the Protista of Haeckel, may have the 
power, though in a minor degree, of assimilating the elements of water, carbonic acid, 
and ammonia*. It has appeared to us that the nutrition of this animal layer may be 
explained more simply, and that it may even be found to be a necessary complement 
to the received doctrine of the “ balance of organic nature.” All sea-water contains a 
certain quantity of organic matter in solution, or in a state of minute molecular subdi- 
vision. This organic matter is derived from the life (by exosmosis) and the death (by de- 
composition) of a fringe of sea-weeds round all coasts ; of a superficial layer of diatoms and 
allied forms covering all the warmer seas ; in the Atlantic of a meadow of sea-weed two 
millions of square miles in extent ; from the life and death of myriads of pelagic and littoral 
animals ; and from rivers, especially the great rivers of the tropics, whose water is con- 
stantly loaded with organic matter. These ternary and higher compounds already gained 
from the inorganic kingdom, and which would otherwise accumulate indefinitely, might 
indeed be dissipated by gradual decomposition, but it seems more consistent with the 
economy of nature that a considerable portion at all events of the organic matter should 
pass through the complete cycle of organic life, in the ocean as well as on the land. 
* Dr. G. C. Wallich, ‘ The North- Atlantic Sea-bed.’ Part 2. The Bathymetrical limit of Animal life in 
the Ocean, p. 131. 
The Monthly Microscopic Journal, January 1, 1869, p. 32. 
