SEA MUSSEL MYTILUS EDULIS. 213 



found present to the amount of 7.5 per cent and pentosan to the amount of 8 to 9 per 

 cent. No fat determination was made. When eelgrass is treated with pancreatin, from 

 23 to 26 per cent of the nitrogen is digested. Since the eelgrass contains 7.5 per cent 

 proteins, of which about one-fourth is digestible by pancreatin, the amount of digestible 

 proteins contained may be put at 1.08 per cent. Decomposed eelgrass contains less 

 nitrogen and is less digestible. For example, black eelgrass (dead) was found to contain 

 1.39 per cent nitrogen, of which but 6.6 per cent was digestible. These figures should, 

 however, in all probability mainly be taken as minimal. 



Experiments on the digestible nitrogenous compounds in the sea bottom brought 

 out the fact that there is only a very small amount of proteins in the bottom which is 

 digestible with pancreatin. In fact, the amount is so small as to be very nearly within 

 the limits of possible error. The analyses for the top layer, however, give such positive 

 results that it is justifiable to conclude that the uppermost layer of the bottom really 

 does contain a certain amount of proteins digestible by pancreatin. In the upper layer 

 from 44 to 68 mg. of digestible proteins per 100 square cm. are found, which means 

 that the amount of digestible proteins per square meter is approximately 5 g. 



On the other hand, digestible nonnitrogenous compounds in the sea bottom consist 

 of a fairly considerable amount of material in the form of pentosans, amounting to from 

 0.3 to 1.0 per cent. This is an important fact, for there is reason to suppose that the 

 bottom fauna is able to digest pentosan. It has been well established that herbivorous 

 animals utilize pentosan as a food, and Biederman and Moritz (1899) showed that 

 gastropods were able to digest pentosan. It is probable, therefore, that bivalves also 

 can digest pentosan and that the considerable amount of pentosan present in the sea 

 bottom besides other possible substances (hemicellulose generally) plays an important 

 part as nonnitrogenous nourishment for a great portion of the bottom fauna. 



In support of Jensen's observations, Blegvad (1914) has made an interesting study 

 of the food of the commonest and most widely distributed bottom-inhabiting animals 

 in the various communities of the Danish waters. His report is based on the analysis 

 of stomach contents. Three main sources of nourishment for the bottom fauna of the 

 sea were determined: (1) Plants — fresh growing plants of the benthos formation, 

 chiefly eelgrass which in the Danish waters produces about 8,232,000 kilograms annually. 

 In course of time, this decays and falls to pieces, forming (2) detritus. This includes 

 dead or dying organisms or portions of them, whether vegetable or animal in origin, as 

 are found in suspension (or solution in the sea water) or deposited upon the bottom. 

 Most of this detritus is of eelgrass origin. (3) Animal or carneous food, or the third 

 source, includes all living animals found in the sea, together with their carrion, save 

 where these are to be reckoned as forming part of the detritus as just defined. 



The plankton, heretofore considered as of greatest significance, he does not list as 

 an important source of food. Whereas previous observers have emphasized the great 

 importance of plankton, Blegvad emphasizes the importance of detritus. He further- 

 more questions Putter's (1908) theory to the effect that the carbon compounds present 

 in solution in the sea water are of very extensive importance as food for certain animals 

 of the bottom fauna. At least it must for the present be regarded as unproved. It is 

 possible, however, that some organisms may live on dissolved organic matter, and so 

 for the sake of convenience Blegvad classifies dissolved organic matter under detritus. 



