FOOD-MATTERS IN THE SEA 313 



(2) the Diatoms,^ minute unicellular plants with siliceous 

 coverings, much smaller than the Copepoda and of a totally 

 different nature, and probably not so suitable for food in 

 the case of a higher animal such as a fish, but available 

 as good vegetable food for many lower invertebrate animals. 

 The Copepoda (being animals) feed upon the Diatoms and 

 other allied minute organisms. The Diatoms, being plants, 

 are, however, able to nourish themselves and build up their 

 bodies from the carbon-dioxide and the soluble salts and 

 other substances dissolved in sea-water. Diatoms are there- 

 fore one of the producing groups in the sea, being able to pro- 

 duce or build organic matters such as starch and protoplasm 

 from inorganic materials ; while Copepoda are consumers, 

 as they require and use up already formed organic matter 

 (such as the Diatoms) for their nutriment. Bacteria 

 (plants without chlorophyll) in the sea are intermediate in 

 this respect. They no doubt require organic food, but 

 probably obtain it from dissolved organic matter derived 

 from sewage and the washings of the land, and from any 

 decomposing animal or vegetable matters in the sea, and 

 other products of the metabolism of higher organisms. 

 Such dissolved organic matter must vary in amount very 

 greatly in different places and in different circumstances, 

 and although constantly renewed it is also constantly being 

 used up or broken down by bacterial action into inorganic 

 matters. It is quite reasonable to suppose that many 

 minute and simple organisms in the sea which have no 

 mouth or other mechanism for taking in solid food, may be 

 able to obtain nutriment from the dissolved organic matter 

 in the water. It may therefore be said that the sea is to 

 some slight extent a nutritive medium, as was pointed out 

 long ago by Dr. W. B. Carpenter ; but very different views 

 have been expressed of late years as to the amount of such 

 possible source of nutriment in the form of dissolved organic 



* There are other still smaller organisms in sea-water, but the 

 Diatoms may be taken as a type of all the micro-phyto-plankton. 



