CH. X] THE CONDITIONS OF LIFE IN THE SEA 233 



not originate by a process of combustion but by the splitting up 

 of the carbohydrate. Doubtless this is, to some extent at least, 

 the case also in the metabolic processes which we have just been 

 discussing, that is the sponge and cucumarian obtain a part of 

 their energy b}^ making use of a process of splitting of the carbon 

 compounds absorbed. Of course oxidation does also take place, 

 but it is subordinated to the fermentation reactions. 



In the case of the sponge we can easily see that there may be 

 a considerable amount of absorption of dissolved food-stuffs in the 

 process of alimentation. In these organisms there is hardly any 

 differentiation of the tissues into organs of digestion, respiration, 

 and so on ; and the whole internal surface of the body of the 

 animal must function as an organ of food absorption and respira- 

 tion. But in the case of a holothurian there is a well-marked 

 alimentar}^ canal and respiratory organs. So also in the case of 

 such organisms as molluscs and ascidians, and one may ask, what 

 are the uses of these organs if the animals feed by absorbing 

 dissolved nutriment, and if the respiratory function is much less 

 important than it is in the case of a warm-blooded animal ? The 

 ctenidia of a mollusc, and the gills of an ascidian, are usually 

 assumed to be respiratory organs, but it is quite certain that a 

 very important function of these structures is that of setting up 

 the current of water which enters into, and passes through the 

 cavities of the animal's body. Now if it is the case that the 

 oxidation process is less important in the metabolism of these 

 animals than is the fermentation reaction, we encounter a difficulty, 

 for why should such an abundant supply of oxygen (in the water 

 current) be necessary to an animal which apparently requires less 

 of this element than those higher in the scale, and which is almost 

 entirely sedentary in its habits. Since the so-called respiratory 

 surface of the mollusc or ascidian is a very large one we may 

 conjecture that it has some other purpose ; and Putter's sugges- 

 tion that it is a surface for the absorption of dissolved food-stuffs 

 appears to be a probable one. At any rate the actual evidence 

 that the gills of these creatures are organs which function in the 

 absorption of dissolved food-stuff is just as strong (or as weak) 

 as the actual evidence that they take up dissolved oxygen from 

 solution in the sea water. 



