412 ME. A. W. ALLEN ON THE 



At this stage, and iu the majority of positions, the young shell 

 runs the risk of being washed awaj', and its existence ended by 

 being smothered in silt. To guard against such a contingency 

 hapjiening whilst the bivalve is unattached, a minute thread, 

 similar in appearance to a strand from a spider's weh, is during 

 its progress paid oat. This thread is strong enough to sustain 

 the weiglit of the oyster, and by its aid it is enabled, if necessary, 

 to regain its lost footing. I did not for some time suspect the 

 existence of this thread, as it is invisible except under the closest 

 observation. However, whilst I was noticing some movements 

 of spat placed on a suspended slab of cement in a small aquarium, 

 I found that the spat travelled to the edge of the slab and, in 

 some cases, succeeded in reaching and anchoring itself on the 

 under surface (away from the light) without mishap. Others, 

 however, were not so fortunate, and on reaching the edge 

 toppled over, but, to my surprise, did not fall to the bottom of 

 the tank, but remained a fraction of an inch, or so below the 

 slab, suspended, as I then discovered, by means of the minute 

 thread referred to above. To regain its position on the slab was 

 then a comparatively easy matter for the oyster. I think it 

 probable that the presence of these minute threads may explain 

 the fact that these shells are so often found grouped together 

 and even anchored on one another. 



As soon as a suitable locality has been found, the shell secures 

 itself by means of its byssus, an attachment which at first 

 consists of a single strand, adhesion being effected by means of 

 a disc-like expansion of its extremity. Other strands are fixed 

 in all directions as the shell grows older, and the resultant 

 bundle is the means of the oyster maintaining an upright and 

 rigid position. The strands are so firmly attached to the 

 anchorage that, if the oyster is forcibly torn away, the fracture 

 occurs at the junction of the byssus and the oyster. Judging 

 from the numbers of bundles observable on dead coral &c., it is 

 probable that such a thing is of frequent occurrence under 

 natural conditions. A new byssus is, however, always formed, 

 the time taken for preliminary re-attachment being only an 

 hour or so. Power of independent movement is, however, 

 confined to shells of six months of age and under; and doubtless 

 tlie possibility or tlie necessity of locomotion grows less and less 

 as the shell grows older and its position more secure. This fact 

 would be sufficient to account for the atrophy of the foot during 



