290 THE ATLANTIC. [chap. v. 



ure and mode of nutrition that they should do so. What may 

 be their extreme limit I am not prepared to say; some sti-ag- 

 gling plants may occur at much greater depths, but cei'tainly 

 what is usually understood by vegetation is practically limited 

 to depths under 100 fathoms. Very few of the higher Algae 

 live even occasionally on the surface of the sea. The notable 

 exception is the gulf-weed {Sargassum hacciferum), which scat- 

 ters its feathery islets over vast areas of warm, still water, and 

 affords rest and shelter to the peculiar nomadic fauna to which 

 I have already alluded (vol. i., p. 180, etc.). 



Confervoids and unicellular Algie occur, however, frequently, 

 and sometimes in such profusion as to discolor the Avater over 

 an area of many miles. If Diatoms are to be regarded as 

 plants, these are found abundantly on the surface, more partic- 

 ularly where the specific gravity of the water is comj)aratively 

 low. The frustules of Diatoms occur in all the deep-sea de- 

 posits in greater or less number; and in some places, as at a 

 few of the stations in the Indian Ocean, they form the bulk of 

 the sample brought up by the sounding -machine. Over the 

 area occupied by this siliceous deposit, the higher fauna were 

 found to consist mainly of forms with but little carbonate of 

 lime entering into the composition of their tests, such as very 

 thin- shelled irregular urchins, and especially an abundance of 

 Tlolothuridea. These were often modified in a singular way ; 

 the perisom was reduced to a mere membrane, and the stomach 

 and intestine were expanded so as to occupy nearly the whole 

 of the body-cavity ; and distended with the " diatom ooze " so 

 completely that the animal looked like a thin transparent bag 

 filled with it. There can be little doubt that the diatoms sink 

 to the bottom still retaining a small portion of their organic 

 matter, which is slowly extracted by the alimentary canal of the 

 Ilolothurid. 



Radiolarians were met with throughout the whole of the At- 

 lantic ; and often in great abundance, the sea being not unfre- 



