FRESH-WATER MUSSELS. 87 



The entire epithelium touching the branchial chamber is abundantly supplied with glands which 

 secrete a mucous substance. The mucus envelops and binds together in strands the material to be 

 transported by the cilia. This is particularly trxie of those particles which are of a very distasteful 

 nature. * * * 



Observers have differed widely in their notions of the ability of the mussel to select its food. To 

 me it is evident that there are, to summarize, four points where such choice is exercised: 



(i) The labial palps, at the upper margin. 



(2) The labial palps, on the furrowed surfaces. 



(3) The mouth. 



(4) The incurrent siphon. 



As to the last, it is surromided by a row of pointed, fleshy papilla, having a resemblance to plant 

 structtu'es. These have two sensory functions — tactile and gustator>- ; for upon being disturbed mechan- 

 ically they are withdrawn into the shell, while a continued teasing, or a strong chemical stimulus results 

 in the closing of the shell. 



Allen conducted experiments the results of which indicated that a mussel siphons 

 a liter of water (about i quart) in approximately 42 minutes. From other observations 

 he was led to infer that mussels pass food through the digestive system somewhat auto- 

 matically or regardless of appetite, but that the secretion of digestive juices and the 

 utilization of the food ingested may be controlled according to the needs of the mussel. 



Allen gives a list of diatoms, desmids and other algae, and miscellaneous food items, 

 but without quantitative data or appraisal of the relative values of the different sorts 

 of food and without reference to the presence of plant detritus in the stomachs. Seem- 

 ingly he supposed, as did many others before him, that mussels subsisted almost exclu- 

 sively upon living organisms. Data bearing on this question are presented in the 

 following section. 



FOOD OF MUSSELS. 

 SIGNIFICANCE OF THE PROBLEM. 



The fact that the rate of growth of mussels seems so directly proportionate to the 

 thickness of the shell (p. 129), or, speaking from a physiological point of view, to the 

 mineral requirements of the mussel — for the shell is chiefly mineral — leads naturally 

 to the supposition that the limiting factor of growth is not the organic food supply, 

 but the mineral food supply. This is a rather startling inference, since we are accustomed 

 to view animals in nature as engaged in a fierce competition for food, their numbers and 

 the luxuriance of growth being proportioned to the abundance of food available; and 

 the food we ordinarily think of is the organic (animal and vegetable) substance required 

 rather than the mineral matter. Yet, if it could be assumed that the food requirements 

 of a floater mussel are of the same nature as those of a pimple-back, theti, since in the 

 same body of water the floater with its shell of paperlike thickness may attain a length 

 of 3K inches in two seasons, while the pimple-back with thick shell may not in the same 

 period attain a length of more than about an inch, the conclusion would seem probable 

 that the thick-shelled species was restricted in growth, not for deficiency of organic 

 food, but for lack of the materials necessary for the formation of shell. The assumption 

 proposed, \iz, that the food requirement of the different species is virtually identical, 

 although plausible and substantiated by some evidence, can not be accepted as finally 

 proved. 



It becomes of importance to determine what is the food of fresh-water mussels, 

 whether the requirements of different species are the same, whether there is serious 

 competition for organic food between commercial and noncommercial species, and 



