第23章

The depth to which worms penetrate, and the construction of their burrows.--Although worms usually live near the surface, yet they burrow to a considerable depth during long-continued dry weather and severe cold.In Scandinavia, according to Eisen, and in Scotland, according to Mr.Lindsay Carnagie, the burrows run down to a depth of from 7 to 8 feet; in North Germany, according to Hoffmeister, from 6 to 8 feet, but Hensen says, from 3 to 6 feet.This latter observer has seen worms frozen at a depth of 1.5 feet beneath the surface.I have not myself had many opportunities for observation, but I have often met with worms at depths of 3 to 4 feet.In a bed of fine sand overlying the chalk, which had never been disturbed, a worm was cut into two at 55 inches, and another was found here at Down in December at the bottom of its burrow, at 61 inches beneath the surface.Lastly, in earth near an old Roman Villa, which had not been disturbed for many centuries, a worm was met with at a depth of 66 inches; and this was in the middle of August.

The burrows run down perpendicularly, or more commonly a little obliquely.They are said sometimes to branch, but as far as I have seen this does not occur, except in recently dug ground and near the surface.They are generally, or as I believe invariably, lined with a thin layer of fine, dark-coloured earth voided by the worms; so that they must at first be made a little wider than their ultimate diameter.I have seen several burrows in undisturbed sand thus lined at a depth of 4 ft.6 in.; and others close to the surface thus lined in recently dug ground.The walls of fresh burrows are often dotted with little globular pellets of voided earth, still soft and viscid; and these, as it appears, are spread out on all sides by the worm as it travels up or down its burrow.The lining thus formed becomes very compact and smooth when nearly dry, and closely fits the worm's body.The minute reflexed bristles which project in rows on all sides from the body, thus have excellent points of support; and the burrow is rendered well adapted for the rapid movement of the animal.The lining appears also to strengthen the walls, and perhaps saves the worm's body from being scratched.I think so because several burrows which passed through a layer of sifted coal-cinders, spread over turf to a thickness of 1.5 inch, had been thus lined to an unusual thickness.In this case the worms,judging from the castings, had pushed the cinders away on all sides and had not swallowed any of them.In another place, burrows similarly lined, passed through a layer of coarse coal-cinders, 3.5 inches in thickness.We thus see that the burrows are not mere excavations, but may rather be compared with tunnels lined with cement.

The mouths of the burrow are in addition often lined with leaves; and this is an instinct distinct from that of plugging them up, and does not appear to have been hitherto noticed.Many leaves of the Scotch-fir or pine (Pinus sylvestris) were given to worms kept in confinement in two pots; and when after several weeks the earth was carefully broken up, the upper parts of three oblique burrows were found surrounded for lengths of 7, 4, and 3.5 inches with pine- leaves, together with fragments of other leaves which had been given the worms as food.Glass beads and bits of tile, which had been strewed on the surface of the soil, were stuck into the interstices between the pine-leaves; and these interstices were likewise plastered with the viscid castings voided by the worms.The structures thus formed cohered so well, that I succeeded in removing one with only a little earth adhering to it.It consisted of a slightly curved cylindrical case, the interior of which could be seen through holes in the sides and at either end.The pine- leaves had all been drawn in by their bases; and the sharp points of the needles had been pressed into the lining of voided earth.Had this not been effectually done, the sharp points would have prevented the retreat of the worms into their burrows; and these structures would have resembled traps armed with converging points of wire, rendering the ingress of an animal easy and its egress difficult or impossible.The skill shown by these worms is noteworthy and is the more remarkable, as the Scotch pine is not a native of this district.

After having examined these burrows made by worms in confinement, I looked at those in a flower-bed near some Scotch pines.These had all been plugged up in the ordinary manner with the leaves of this tree, drawn in for a length of from 1 to 1.5 inch; but the mouths of many of them were likewise lined with them, mingled with fragments of other kinds of leaves, drawn in to a depth of 4 or 5 inches.Worms often remain, as formerly stated, for a long time close to the mouths of their burrows, apparently forwarmth; and the basket-like structures formed of leaves would keep their bodies from coming into close contact with the cold damp earth.That they habitually rested on the pine-leaves, was rendered probable by their clean and almost polished surfaces.