SEA MUSSEL MYTILUS EDULIS. 207 



presence of a golden-brown pigment called diatomin. In addition to these structures, 

 one or more conspicuous oil droplets are often visible within the cell. 



The function of the chromatophores is a subject that deals with one of the most 

 fundamental principles of marine food supply, for it is through these bodies that the 

 plant is able, in the presence of sunlight, to convert inorganic materials into organic 

 compounds which may be utilized as food. 



The chemical elements required for the nourishment of the plant are carbon, hydro- 

 gen, oxygen, nitrogen, sulphur, phosphorus, calcium, silicon, iron, and chlorine. If all of 

 these elements were present in the ocean in unlimited quantities there would be no limit 

 to the quantity of plankton organisms that might be produced. The limited presence of 

 a single one of these elements, however, is sufficient to limit plankton production. 

 According to von Leibig's "Law of the Minimum," a plant requires a certain number of 

 foodstuffs if it is to continue to live and grow, and each of these food substances must 

 be present in a certain proportion. If one of them is absent the plant will die; and if 

 it is present in minimal proportion the growth will also be minimal. This will be the 

 case, no matter how abundant the other foodstuffs may be. With the exception of 

 nitrogen, silicon, and phosphorus compounds, the foodstuffs necessary for the support 

 of plants are exceedingly abundant in the sea. The quantity of marine plants, there- 

 fore, fluctuates in relation to the proportions of these rarer but indispensable foodstuffs. 

 The water of the warmer seas is lower in its nitrogen content than that of the colder 

 seas, and in accord with these conditions we find a much richer plankton population in 

 the latter region. Brandt (1898) showed that the lakes which Apstein (1896) found 

 richest in plankton also contained the greatest amount of inorganic nitrogen. In the 

 Bay of Kiel, where silicic acid was proportionately the least abundant of the required 

 foodstuffs, Raben (1905) found that the increase and decrease in the number of diatoms 

 ran parallel with the amounts of silicic acid present. 



The myriads of diatoms scattered throughout the sea represent so many chemical 

 laboratories in which the solar energy is utilized to combine the air, water, and salts of 

 the sea into the three food principles — proteins, fats, and carbohydrates — upon which 

 all animals are dependent. This is one reason why they have been considered as im- 

 portant for the support of the animals of the sea as the grasses, vegetables, and fruits 

 are for the terrestrial fauna. 



Diatoms are peculiar in that many of them possess the power of movement. They 

 may glide slowly over a solid substratum or over moist surfaces which serve as a fulcrum 

 for movement. The direction of motion is usually along a more or less curved path 

 and may be reversed. How the locomotor energy is developed was explained by Siebold 

 in 1849, who demonstrated a streaming movement of external protoplasm which under- 

 goes a periodic reversal of direction. This was shown by the fact that particles of sand 

 or indigo adhering to the upper valve of a fixed diatom are moved alternately back- 

 ward and forward from one pole to the other. It has more recently been shown by 

 O. Miiller that the protoplasm exudes through the polar furrow on each of the valves, 

 streams along the crevice of the raphe to its termination at the median nodule, where 

 each stream returns to the interior, and travels back internally. The result of the 

 movement of these extracellular masses of protoplasm is to create friction against the 

 surrounding media and cause a forward movement of the organism in the opposite 

 direction. The extracellular layer of protoplasm is extremely thin, and if it moves at 



