186 Mussel Beds; their Profluctivitu and Ma'mtenaiwe 



entering the mouth of the tow-net, pass out again through the meshes 

 of the silk or other fabric of which the net is fashioned. As the food 

 supply is ample, we are forced to conclude that, for some reason, the 

 individuals are not always able to avail themselves of it. 



Silting of the Beds. 



It has been said in the foregoing that parts of the mussel nursery 

 situated towards the Bar at Aberdovey appear to be always covered by 

 sand to a greater or less depth, — not necessarily the same portions all 

 the time. I have never visited this place without observing patches of 

 it in this condition, the buried mussels lying beneath from one to about 

 four inches of sand. As this place dries only at exceptionally low spring 

 tides, when the volume of water entering and leaving the estuary is 

 much greater than normal, we may infer that this "sanding" process is 

 worse at such times than during neap tides. Surface tow-nettings, taken 

 at various states of the tide, reveal the large amount of sand always 

 carried in suspension by the current, both at Aberdovey and in the other 

 estuaries. On the other hand, the sand probably remains longer over 

 any given place during the lesser tides than during springs, when patches 

 covered by one tide may be cleared by the next. The direction and 

 force of the wind are other factors which have considerable influence in 

 this silting process. From some preliminary (incomplete) experiments 

 which I carried out at Aberdovey, under artificial conditions, during the 

 summer of 1916, I found that mussels, despite all care, succumbed in 

 from four to five days when covered by sand. Not only was this the 

 case, but, with even the slightest covering of this material, the mussels 

 made no effort whatever to raise themselves above it, or otherwise to 

 free themselves. Yet a vigorous stream from the exhalent siphon would 

 have cleared them, as the individuals were in a position favourable to 

 such action. This dilemma seems to be one in which the foot might 

 be employed with advantage if it is capable of being utilised as an aid to 

 movement in the adult stage. I have not observed it to be used for this 

 purpose, except in very small individuals, at any time. Although diffi- 

 cult of proof, I am inclined to the view that Mytilus behaves in a similar 

 manner when covered by sand in natural conditions. That is, it remains 

 passive, with its normal life processes slowed down, until the sand is 

 removed bv the current, or till death supervenes. Fishermen often 

 assert that mussels are able to raise themselves above accumulating 

 mud or sand by the lengthening of their byssus strands, which are at- 

 tached to underlying stones and other substances. What really happens 



