•266 
THE VOYAGE OF H.M.S. CHALLENGER. 
Brackiopoda . — These organisms are found living even in the greatest depths of the 
ocean ; occasionally they are dredged in large numbers in depths down to 300 fathoms, 
but in deep water it is only rarely that their remains can be detected in the deposits.^ 
Pteropoda and Heteropoda . — A large number of these pelagic Molluscs secrete 
carbonate of lime shells, and this is especially the case in tropical waters. In polar 
regions the place of the shelled species is taken, with the exception of one or two small 
species of Limacina, by shell-less species. The shells of the tropical species make 
up a large part of some tropical and subtropical deposits from moderate depths, in which 
there is a relatively small quantity of land debris. Like the pelagic Foraminifera, these 
]>elagic iMollusca attain their greatest development in the warm oceanic currents, and 
diminish both in the numl^er of species and the size and mass of the shells as the colder 
currents of the polar regions are approached. Like the pelagic Foraminifera, also, the 
distribution of the living animals at the surface corresponds with the distribution of their 
dead shells over the sea-bed, with certain limits as to depth. The dead shells are not 
universally distributed over the floor of the ocean, for in all the deposits from the greater 
depths of the ocean they are absent, or only rare fragments are met with, and as a general 
rule they disappear from deep-sea deposits with increasing depth in the same way as the 
shells of pelagic Foraminifera, the more delicate and fragile ones being found only in 
the lesser depths. In the deposits of polar regions these shells are very rarely, if ever, 
observed in the deposits, and certainly never make up any sensible part of the carbonate 
of lime in the muds or oozes. A list of the species, whose shells may constitute a large 
part of a Pteropod Ooze, is given on page 224.^ 
The Pteropoda and Heteropoda live in the surface and subsurface waters of the ocean, 
are Holoplanktonic, and belong exclusively to the pelagic Plankton. It has never been 
suggested that they lived exclusively, or for any portion of their lives, at the bottom 
of the sea, as was long maintained with reference to the pelagic Foraminifera. It is 
interesting then to point out that the shells of these pelagic Molluscs follow the same 
order witli respect to distribution in depth as the shells of pelagic Foraminifera. They 
are abundant, and the shells of all species appear to be represented, in the shallower 
deposits, but witli increasing depth the more delicate shells first disappear, and then the 
thicker and more massive ones. In depths of 2300 fathoms they are wholly removed 
from the deposits, or only an occasional fragment is encountered. In the surface waters, 
however, the living animals are quite as abundant over the region where the shells are 
absent, as over the region where they are present, on the bottom. In whatever way we 
may account for the removal of tlie Ptcro 2 >od shells from the deeper deposits of the ocean, 
the same rea.soning is evidently applicable to the removal of the shells of pelagic Foramini- 
• See Davidson, Report on the Brachiopoda, Zool. Chall. Exp., pt. 1. 
• Sec Pelscneer, Report on the Pteropoda, Zool. Chall. Exp., ]>t. 65 ; Smith, Report on the Heteropoda, Zool. 
Chall. Exp., pt. 72. 
