566 A NATURALIST ON THE "CHALLENGER 



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such as the Pteropods, Ctenaphora and SiphonopTwra, belong to 

 groups peculiar to the sea surface, and have, no doubt, a most 

 ancient connection with it, whilst others are forms, the more 

 immediate progenitors of which lived a terrestrial or littoral 

 existence, and which, having taken to Pelagic habits, have become 

 modified only in less important particulars of their structure 

 to suit their new habits of life. 



The surface water of the open ocean is full of vegetable life. 

 Diatoms are to be found with the surface net everywhere, and 

 in high northern and southern latitudes* they abound extremely, 

 so as to colour the ice with their debris, change the tint of the 

 water, fill the towing net up with slimy masses and cover the 

 deep-sea bottom with a silicious deposit of their skeletons. 



In tropical seas, other lowly organized algae especially 

 abound-; mainly Oscillator ice, of the genus Trichodesmium. These 

 algae occur in the water as small brown faggots of minute 

 threads, resembling, as Mr. Berkeley says, minute fragments of 

 chopped hay. Together with these forms others often occur in 

 which the threads are gathered into small globular masses 

 with the ends of the threads all directed outwards. When 

 tracts of the sea are passed through, which are full of this 

 Trichodesmium, the water lighted up by sunlight, when looked 

 down into, appears as if full of small particles of mica, or some 

 such substance, so strongly is the light reflected from the minute 

 bundles of the algae. 



We met with this alga in greatest abundance in the Arafura 

 Sea, between Torres Straits and the Am Islands. Here it was at 

 first encountered discolouring the sea-surface in bands and 

 streaks ; as the ship moved farther on, it became thicker, and at 

 length the whole sea, far and wide, was discoloured with it. It 

 remained still, however, denser in long streaks, and within these 

 again it was massed in small patches. There was a strong smell 

 from these patches, as from a pond covered with vegetation. So 

 abundant is Trichodesmium in some seas, that one of the explana- 

 tions of the name of the Eecl Sea is that the term was derived 

 from the discolouration of the water by vast quantities of 

 Trichodesmium Erythrccum. 



* See page 249. 



