第21章

Means by which worms excavate their burrows.--This is effected in two ways; by pushing away the earth on all sides, and by swallowing it.In the former case, the worm inserts the stretched out and attenuated anterior extremity of its body into any little crevice, or hole; and then, as Perrier remarks, the pharynx is pushed forwards into this part, which consequently swells and pushes away the earth on all sides.The anterior extremity thus serves as a wedge.It also serves, as we have before seen, for prehension and suction, and as a tactile organ.A worm was placed on loose mould, and it buried itself in between two and three minutes.On another occasion four worms disappeared in 15 minutes between the sides of the pot and the earth, which had been moderately pressed down.On a third occasion three large worms and a small one were placed on loose mould well mixed with fine sand and firmly pressed down, and they all disappeared, except the tail of one, in 35 minutes.On a fourth occasion six large worms were placed on argillaceous mud mixed with sand firmly pressed down, and they disappeared, except the extreme tips of the tails of two of them, in 40 minutes.In none of these cases, did the worms swallow, as far as could be seen, any earth.They generally entered the ground close to the sides of the pot.

A pot was next filled with very fine ferruginous sand, which was pressed down, well watered, and thus rendered extremely compact.A large worm left on the surface did not succeed in penetrating it for some hours, and did not bury itself completely until 25 hrs.40 min.had elapsed.This was effected by the sand being swallowed, as was evident by the large quantity ejected from the vent, long before the whole body had disappeared.Castings of a similar nature continued to be ejected from the burrow during the whole of the following day.

As doubts have been expressed by some writers whether worms ever swallow earth solely for the sake of making their burrows, some additional cases may be given.A mass of fine reddish sand, 23 inches in thickness, left on the ground for nearly two years, had been penetrated in many places by worms; and their castings consisted partly of the reddish sand and partly of black earth brought up from beneath the mass.This sand had been dug up from a considerable depth, and was of so poor a nature that weeds could not grow on it.It is therefore highly improbable that it should have been swallowed by the worms as food.Again in a field near my house the castings frequently consist of almost pure chalk, which lies at only a little depth beneath the surface; and here again it is very improbable that the chalk should have been swallowed for the sake of the very little organic matter which could have percolated into it from the poor overlying pasture.Lastly, a casting thrown up through the concrete and decayed mortar between the tiles, with which the now ruined aisle of Beaulieu Abbey had formerly been paved, was washed, so that the coarser matter alone was left.This consisted of grains of quartz, micaceous slate, other rocks, and bricks or tiles, many of them from 1/20 to 1/10 inch in diameter.No one will suppose that these grains were swallowed as food, yet they formed more than half of the casting, for they weighed 19 grains, the whole casting having weighed 33 grains.Whenever a worm burrows to a depth of some feet in undisturbed compact ground, it must form its passage by swallowing the earth; for it is incredible that the ground could yield on all sides to the pressure of the pharynx when pushed forwards within the worm's body.

That worms swallow a larger quantity of earth for the sake ofextracting any nutritious matter which it may contain than for making their burrows, appears to me certain.But as this old belief has been doubted by so high an authority as Claparede, evidence in its favour must be given in some detail.There is no a priori improbability in such a belief, for besides other annelids, especially the Arenicola marina, which throws up such a profusion of castings on our tidal sands, and which it is believed thus subsists, there are animals belonging to the most distinct classes, which do not burrow, but habitually swallow large quantities of sand; namely, the molluscan Onchidium and many Echinoderms.

If earth were swallowed only when worms deepened their burrows or made new ones, castings would be thrown up only occasionally; but in many places fresh castings may be seen every morning, and the amount of earth ejected from the same burrow on successive days is large.Yet worms do not burrow to a great depth, except when the weather is very dry or intensely cold.On my lawn the black vegetable mould or humus is only about 5 inches in thickness, and overlies light-coloured or reddish clayey soil: now when castings are thrown up in the greatest profusion, only a small proportion are light coloured, and it is incredible that the worms should daily make fresh burrows in every direction in the thin superficial layer of dark-coloured mould, unless they obtained nutriment of some kind from it.I have observed a strictly analogous case in a field near my house where bright red clay lay close beneath the surface.Again on one part of the Downs near Winchester the vegetable mould overlying the chalk was found to be only from 3 to 4 inches in thickness; and the many castings here ejected were as black as ink and did not effervesce with acids; so that the worms must have confined themselves to this thin superficial layer of mould, of which large quantities were daily swallowed.In another place at no great distance the castings were white; and why the worms should have burrowed into the chalk in some places and not in others, I am unable to conjecture.