26 FEEDING 



of diatoms that live near the surface of the sea is shown by 

 the fact that their skeletons cover an area of about 10,880,000 

 square miles of the bottom of the ocean, principally in the 

 southern waters. In one haul, which traversed two cubic 

 metres of sea, there were no less than 3,173,000,000 diatoms 

 taken; and yet it is believed that two-thirds of those present 

 when the collection was taken escaped through the meshes of 

 the fine net. Observations made in the Channel during the late 

 winter months of sunshine on the number of diatoms enable 

 us to some extent to foretell the success or othermse of the 

 mackerel fisheries of the later months of the year. 



Besides the diatoms there are innumerable other unicellular 

 plants and animals and an immeasurable number of minute 

 larvae of animals floating about in the sea and in our rivers 

 and lakes. Some of these are so minute that they readily 

 slip through the meshes of the finest fabric man can produce. 

 Fortunately Nature has come to the rescue, and the feeding 

 organs of a certain sea-squirt, an Ascidian, Oikopleura by 

 name, net the very minute floating organisms which cannot 

 be caught otherwise, though they may be separated in bulk 

 by using a centrifuge. The floating and, for the most part, 

 unicellular plants are collectively known as Phytoplankton, 

 and the Phytoplankton of the sea, though composed of minute 

 plants, is far vaster in quantity and bulk than all the littoral 

 or shore seaweeds put together: the latter are relatively un- 

 important as a source of food for sea animals. 



Diatoms are far more numerous in Arctic and Antarctic 

 regions than in the tropical seas and with this is associated 

 the enormous abundance and the colossal size of the circum- 

 polar marine life. A certain jelly-fish, Cyanea arctica, is said 

 to attain a diameter of thirty feet with tentacles 200-300 feet 

 long. Also a certain cuttle-fish, LoUgo, may become eighteen feet 

 or more in length, whilst its allied forms from temperate seas 

 are but some three inches long. Crustacea allied to our sand- 

 hopper attain in these polar regions the dimensions of lobsters. 



Bacteria 



There is another group of lowly plants, microscopic in size, 

 which are in many ways indispensable to the life of higher 



