REPORT ON THE DEEP-SEA DEPOSITS. 
257 
d. Calcaeeous Organic Kemains in Deep-Sea Deposits. 
Calcareous Algse. — Species of Algae which secrete carbonate of lime are abundant in 
the shallow waters of the ocean. In the tropical regions especially there are large and 
massive species of Lithothamnion, Lithophyllum, Halimeda, and other genera that make 
up a large part of some coral reefs and of the surrounding Coral Sands and Muds. Two 
hundred fathoms is probably the extreme limit at which any of these organisms live in 
the ocean, but the broken-down fragments of calcareous Algae have been found in depths 
of over 2000 fathoms in the neighbourhood of coral reefs. In the Tables of Chapter II. 
they are noted in all the Coral Muds and Sands, in six different samples of Globigerina 
Ooze, and in very many samples of Volcanic Muds and Sands. 
Coccospheres and Rhabdospheres. — The precise nature of these minute organisms was 
for a long time obscure, but they are now regarded, and no doubt rightly, as pelagic Algae. 
There is considerable difference in the size and form of both the Coccospheres and Rhabdo- 
spheres ; three of the principal forms are repre- 
sented in the annexed woodcuts. The interior of 
the spheres is filled with transparent albuminoid 
matter, in which no nucleus was detected by the 
Challenger naturalists. When the calcareous rods 
and discs are removed by dilute acid, small 
gelatinous spheres remain behind, on the outer 
surface of which the Coccoliths and Rhabdoliths 
were implanted or embedded. Rhabdospheres 
are especially developed in equatorial and tropical 
regions, and are rarely met with in regions where 
the temperature of the surface water falls below 
65° F. Coccospheres, while abundant in tropical 
waters, are found further north and south than 
the Rhabdospheres ; they are present even where the temperature on the surface is as 
low as 45° F., indeed, Coccospheres attain their greatest development in temperate 
regions. These organisms are absent or rare in coast waters affected by rivers ; they 
especially flourish in the pelagic currents of the open ocean, and therefore belong to the 
pelagic Plankton. In Arctic and Antarctic waters Coccospheres and Rhabdospheres are 
replaced by similar minute Algse, which do not, however, secrete rods and discs of car- 
bonate of lime on their outer surfaces.^ Coccospheres and Rhabdospheres are, then, 
nearly everywhere present in the surface waters of the tropical and temperate regions 
1 Narr. Chall. Exp., vol. i. pp. 436, 938, 939. 
(DEEP-SEA DEPOSITS CHALL. EXP. — 1891.) 
33 
