CH. X] THE CONDITIONS OF LIFE IX THE SEA 225 



by the 3^east and reappears as the vital activity of that organism. 

 The yeast multiplies and in its metabolism the enzymes, or 

 ferments, which are instrumental in the splitting up of the sugar 

 to form alcohol and carbon dioxide, are excreted. The saprophytic 

 plants in the sea are moulds, yeasts and fungi, but they are not 

 abundant. The marine bacteria also exhibit a mode of nutrition 

 which is related to that just discussed, but it will be better that 

 we should consider it separately. 



Myxotrophic animals. We can only draw a rather in- 

 definite line of demarcation between plants and animals. In the 

 case of the higher members of each kingdom there is, of course, 

 a very sharp distinction, both in structure and life-history, and 

 in the general character of the metabolism of the organism. But 

 among the lower animals and plants this distinction disappears 

 and several groups of organisms have at one time been included 

 among the plants, and at another among the animals. Certain 

 well-known characters are distinctive of the plant — the relatively 

 thick cell- wall of cellulose; the presence of chlorophyll bodies in 

 the green parts ; and the general absence of motility. On the 

 other hand the much less strongly developed cell- wall ; the general 

 presence of chitin instead of cellulose ; the absence of chlorophyll ; 

 and the general presence of organs of locomotion, are characteristic 

 of the animal. But we find that investments composed of a sub- 

 stance resembling cellulose may occur among animals, while the 

 cell-walls of bacteria appear to be composed of chitin ^ Chloro- 

 phyll bodies are absent in many plants, and are, apparently, present 

 in many animals. Diatoms, algal spores, and bacteria are actively 

 motile, while some animals, such as the trophic forms of myxo- 

 sporidia, are immotile. There are many species among the fiagellate 

 protozoa which, if we were to consider the morphology alone, we 

 should call animals ; while their mode of nutrition indicates that 

 they belong to the vegetable kingdom. All this confusion need 

 not, however, trouble us. In the sea there are only individuals 

 and whether we call them animals or plants is not of importance 

 in our enquiry. Morphological distinctions, it has been said, are 

 ultimately physiological ones; and, as we are beginning to learn, 

 depend on the almost infinite diversity of conformation of the 

 proteid molecule-complexes which make up their protoplasm. 



1 Mary Leach, Journ. Biol. Chem., 1906, (1), p. 463. 

 J. F. 15 



