28 THE FISHING INDUSTRY 



There are also many larval forms of marine worms 

 and jellyfish, and many kinds of microscopic, unicellular 

 organisms, some of which are vegetable and others are 

 clearly animal. The chief animal forms belong either 

 to the Infusoria, the Foraminifera or the Radiolaria. 

 The shells of the two latter forms accumulate at the 

 bottom of the sea, producing the deposits known 

 as the Globigerina and Radiolarian oozes. In this 

 way, chalk deposits were formed in primitive times. 



The most important vegetable planktonic organisms 

 are the Diatoms. Their accumulated shells form 

 important deep-sea deposits. 



The numerous varieties of planktonic life can thus 

 be divided into two groups : those minute animal and 

 vegetable organisms that pass the whole of their exist- 

 ence at the surface of the sea the true constituents of 

 plankton all the year round and the eggs and larvae 

 of many species of fish that are found among the plankton 

 only at certain times of the year notably in spring and 

 summer. 



The quantity of organic food substances such as 

 albumen, fat and carbohydrate, that is contained in the 

 plankton produced annually by a given area of the sea, 

 has been compared with the quantity of such substances 

 produced by a similar area' of land in crops such as 

 pasture, hay, lupine and peas. In this way, it has been 

 estimated that the productivity of the sea is about 20 

 per cent less than that of average land. 



Unlike that of the land, the productivity of the sea 

 is greater in colder latitudes than in the tropics. This 

 somewhat unexpected fact is attributable to the action 

 of denitrifying bacteria which, flourishing more readily 

 in warm, tropical waters, effectively reduce the amount 

 of available nitrogen compounds in the water. In colder 

 waters, denitrifying bacteria are less active, and nitrates 



