RECENT WORK ON MARINE METABOLISM 15 



manner, but this was apparently considered and regarded as 

 unlikely. It is also possible that changes in the intensity of 

 light may affect the sense organs on the surface of the body, 

 and by producing changes in the amount of movement, so 

 indirectly affect the rate of metabolism. The sponge is a 

 sedentary animal quite incapable of locomotion, and the only 

 mechanical work performed by it appears to be the maintenance 

 of the current of water circulating through the canal system. 

 Possibly light exchanges may affect the velocity of this current, 

 and so indirectly the intensity of metabolism. But the evidence 

 seems to point to a direct influence of changes in the intensity 

 of the incident light on the metabolism of the organism. 



Thus certain conclusions, which appear to me to be quite 

 novel, emerge from a consideration of the observations now 

 under review : (1) that sponges and cucumarians obtain their 

 food otherwise than by capturing and digesting living organisms; 

 (2) that the intensity of metabolism in some sponges, at least, 

 is directly influenced by changes in the intensity of light, 

 so that photo-chemical reactions are carried on in these 

 organisms just as in a green plant ; and (3) that the mode 

 of nutrition of some of the lower invertebrates differs strikingly 

 from that of the higher animals, and is to be compared with 

 that of the saprophytic plants. 



What is the nature of the food substances utilised by the 

 sponge and holothurian, and probably by many other of the 

 lower marine invertebrata, such as ascidians, some molluscs, 

 deep-sea fishes, and probably many other animals ? It appears 

 to be very probable that these food-stuffs are present in solu- 

 tion in the sea in the shape of organic compounds such as 

 extractives, carbohydrates, organic acids, and substances akin 

 to the humus compounds present in the soil. I have already 

 shown that recent investigations indicate that certain inorganic 

 salts, present in solution in the sea, are of first-rate importance 

 for the nutrition of the lower organisms of the plankton, and 

 consequently for the animals which feed upon the latter. We 

 must recognise now that animals may also feed upon substances 

 dissolved in the sea. 



So far there is not much evidence that these organic 

 compounds are universally present in solution in the sea in 

 such proportion as to constitute an abundant food supply 

 for saprozooic animals. Putter has indeed shown that such 



