F. S. WRiaiiT 185 



the forms under discussion, because in them (most Lamellibranchs, 

 Ascidians, etc.) the cilia of the gill epithelium alone serve as the mechanism 

 of the respiratory streams. A large gill-surface means that a larger 

 volume of water can be treated by those forms possessing it. 



Even if the validity of Putter's theory of "osmotic" nutrition be 

 fully conceded, it does not help us much to understand the poor flesh 

 content of the seed mussels. It must be stated again that, as the 

 Ro-ddu Scar is situated near the narrow neck of the estuary, and conse- 

 quently all the water entering or leaving it has to pass over the bed, any 

 such dissolved food material that may be present higher upstream will 

 have to pass here on its way seaward. 



Fouling of the Beds, etc. 



The toxic effect of noxious matters liberated in the course of meta- 

 bolism by large and crowded assemblages of animals, — products of 

 excretion and metabolism, — must now be considered brieflv. This 

 factor may be taken as explaining, in some degree at least, the poor 

 condition and high mortality of mussels in those beds which, besides 

 being very crowded, are situated in a sluggish current, where waste 

 products may remain and accumulate to the detriment of the population^. 

 When the shellfish are several layers deep, such a condition of things 

 will probably be intensified. On the Ro-ddu Scar, however, the animals, 

 though very closely packed, are never overlain by their congeners. 

 Because of this, in conjunction with the great tidal scour to which the 

 place is subjected, the patches of mussels, and the sand in their vicinity, 

 are always remarkably free of noxious substances. Mussel excreta may 

 certainly be detected at times, but in quite neghgible amount. 



Food Material. 



The plentiful and varied nature of the food in the stomach content 

 of mussels from all situations (diatoms, protozoa, detritus, etc.) proves 

 that there is no lack of suitable aliment. Each mussel (as has been 

 explained), in the seed beds will have free access to the water currents, 

 except in certain circumstances to be considered presently. The paucity 

 of diatoms and other organisms, in material collected by means of the 

 tow-net near the mussel beds, compares in an interesting manner with 

 the stomach content of Mytilus, with its wealth of organisms. It must 

 be borne in mind, however, that the greater number of diatoms, etc., 



1 See article by R. L. Ascroft.. "Mussel Beds and Mud Banks." in the Report for 1898 

 of the Lanes and Western Sea Fisheries Laboratory. 



