54 Proceedings of lloyal Society of Edinburgh. [sess. 
Looking then to these, as presenting all the facts npon which we 
must argue in interpreting the past, we find that our foreshore 
annelids are of many kinds and sizes, from one species which 
attains the great proportions indicated by a length of 3 feet to 
other species which are very small. We find that the commonest 
varieties which leave their marks at low tide on almost every 
possible spot of shore, bore and tunnel into the wet sands and mud 
with almost incredible ease and facility of motion. Professor 
Mackintosh, who, as must be well known to the members of this 
Society, has made a special study of these organisms, speaks of 
them as “ dashing ” through open water and through wet sand with 
almost equal ease.* He tells us that these perforations are very 
various in direction and in shape ; that the animal secretes from its 
skin a fine lubricating slime, which must always have a tendency to 
agglutinate the grains of sand that come into contact with it, and 
which in some species is employed to construct a tubular case that 
projects from the surface of the sand, and constitutes a sort of house 
or fortress from which the creature protrudes its tentacles or its 
proboscis for the seizure of its food. But the food of the common 
burrowing sand worms is found in the sand itself, and the agglutin- 
ating power of their slime on the surrounding particles of sand may 
have no other effect than that of so hardening or indurating the inner 
surface of the tubes as to render them comparatively lasting, or, at 
all events, to prevent their speedy obliteration. Here we have one 
obvious explanation of the possibility of fossilisation, or of permanent 
endurance through the processes which convert soft sediments into 
hardened rocks. And then, again, we find that another step forward 
in our investigation of existing facts becomes an important step for- 
ward also in our interpretation of the past. We know that the pur- 
pose or aim of burrowing worms, in the exercise of their extraordinary 
muscular energy in boring, is to extract food from the substance, what- 
ever it may be, in which they burrow. We know further, that this 
food is extracted by the power which the worm has of swallowing the 
finer particles of its surrounding medium, of passing them through 
the whole length of its body, of submitting them there to its digestive 
apparatus, and of discharging behind it all the mere mineral particles 
* “On the Boring of certain Annelids,” Ann. and Mag. Nat. Hist., Oc . 
1868. 
