232 Proceedings of Royal Society of Edinburgh. [sess. 
can be collected in tow-nets as a yellowish slimy mass. Herring- 
fishers are sometimes hampered in their operations by the vast 
floating banks of these Algse. * In the Arafura Sea and other 
tropical and subtropical regions the “ Challenger ” Expedition also 
collected great numbers of Diatoms at the surface, especially where 
there was brackish water, or, at least, water of a relatively low 
salinity. In the true oceanic waters of the tropical regions the 
number of species of Diatoms is probably as numerous as in polar 
waters, but the individuals are not nearly so abundant. In a Diatom 
ooze from lat. 54° S., 48 species of Diatoms were observed in the 
deposit, while in a tropical Eadiolarian ooze, lat. 6° N., 51 species 
were recognised ; of these 14 species are common to the two stations. 
In the former case the Diatom remains make up over 50 per cent, 
of the whole deposit, in the latter not more than 2 or 3 per cent. 
Some large species of tropical Etmodiscus ( Coscinodiscus ) have an 
extremely thin shell of silica, and indeed, all the Diatom frustules 
of species that live in truly pelagic waters of the tropical and sub- 
tropical regions are exceedingly thin and delicate compared with 
those in colder and coast waters. When Diatoms cannot be ob- 
served directly in the tow-net gatherings, they can almost always be 
found in the stomachs of Salpce, Doliolum , and other marine animals. 
The specimens of Diatoms met with in the open sea all belong to 
pelagic species, but it is not uncommon to meet with attached forms 
fixed to the backs of Copepods and other Crustacea, as well as on 
pelagic Molluscs. These siliceous pelagic Algge or Diatoms, together 
with other Algse, some of which secrete carbonate of lime — Cocco- 
spheres and Ehabdospheres — appear to live only in those upper 
layers of oceanic waters that are affected by sunlight, and they are 
the original source of the food of the vast majority of animals living 
at the surface and on the bottom of the sea, for on falling to the 
bottom the Diatoms still retain a portion of their organic matter, 
and thus supply with nourishment those animals, like Echinoderms 
and Annelids, which live at the bottom by eating the mud or ooze 
there in process of accumulation. 
The Eadiolarians, unlike the Diatoms, are rarely met with in any 
numbers in estuaries or near the mouths of rivers, their true habitat 
being in the open ocean ; they belong to oceanic as distinguished 
* Pearcey, Proc. Roy. Phys. Soc. Edin., vol. viii. p. 400. 
