1888-89.] The Duke of Argyll on Bodies of Organic Origin. 55 
which have been stripped of nutriment. In this respect the marine 
worms perform the same work that on land is done hy the earth 
worm. It would seem to follow from this process that one con- 
sequence of it is likely to be that, as the worm passes through the 
sand, the finer grains within reach of its swallowing power will 
he segregated from the coarser, and be concentrated in what are so 
well known as the “castings,” most of which are ejected on the 
surface, hut some of which may very probably remain in portions 
of the burrow. And now we reach a further fact of curious 
significance and interest. We know that all organic matter has 
more or less the property of decomposing the mineral compounds 
of iron, and of reducing them to red oxides. What, then, is the 
probable effect upon such particles of iron as may he ingested 
by the worms, and passed through their bodies? Have we any 
evidence on this point from our living annelids ? Yes, we have. It 
is found, for example, that when a worm burrows through blue or 
dark coloured shale, the castings of the worm, which are the debris 
of the mineral, are not of the same colour as the matrix from which 
they come ; from blue or blackish, they are converted into brownish 
or reddish material.* On the same principle, and by the operation 
of the same causes, we see that the particles of sand which are 
agglutinated by the slime of the annelid, and are converted into a 
thin wall by the coherence thus produced, would probably exhibit 
the same change of colour due to the peroxidation of any iron present 
in the surrounding material. Hence, again, we are led to the 
conclusion that several characteristics may be expected in any marks 
left in rocks by the boring of annelids — first, that the walls may retain 
a special character distinguishable from the matrix ; secondly, that 
one indication of these may probably be some visible development 
of the oxides of iron, more or less conspicuous according to the 
abundance or scarcity of iron in the matrix ; and thirdly, that 
wherever the castings have been left in the tubes, or have been 
washed into them again, they would most probably exhibit some 
difference both of texture and of colour as compared with the matrix. 
There is another conclusion, which we may draw assuredly from 
the series of causes thus traced, and that is a conclusion as to the 
effects which would probably result from the complete breaking up of 
* Mackintosh, Mag. Nat. Hist., Oct. 1868, p. 5. 
