Not only do worms aid directly in the chemical disintegration of rocks, but there is good reason to believe that they likewise act in a direct and mechanical manner on the smaller particles.All the species which swallow earth are furnished with gizzards; and these are lined with so thick a chitinous membrane, that Perrier speaks of it, as "une veritable armature." The gizzard is surrounded by powerful transverse muscles, which, according to Claparede, are about ten times as thick as the longitudinal ones; and Perrier saw them contracting energetically.Worms belonging to one genus, Digaster, have two distinct but quite similar gizzards; and in another genus, Moniligaster, the second gizzard consists of four pouches, one succeeding the other, so that it may almost be said to have five gizzards. In the same manner as gallinaceous and struthious birds swallow stones to aid in the trituration of their food, so it appears to be with terricolous worms.The gizzards of thirty-eight of our common worms were opened, and in twenty-five of them small stones or grains of sand, sometimes together with the hard calcareous concretions formed within the anterior calciferous glands, were found, and in two others concretions alone.In the gizzards of the remaining worms there were no stones; but some of these were not real exceptions, as the gizzards were opened late in the autumn, when the worms had ceased to feed and their gizzards were quite empty.
When worms make their burrows through earth abounding with little stones, no doubt many will be unavoidably swallowed; but it must not be supposed that this fact accounts for the frequency with which stones and sand are found in their gizzards.For beads of glass and fragments of brick and of hard tiles were scattered over the surface of the earth, in pots in which worms were kept and had already made their burrows; and very many of these beads and fragments were picked up and swallowed by the worms, for they were found in their castings, intestines, and gizzards.
They even swallowed the coarse red dust, formed by the pounding of the tiles.Nor can it be supposed that they mistook the beads and fragments for food; for we have seen that their taste is delicate enough to distinguish between different kinds of leaves.It is therefore manifest that they swallow hard objects, such as bits of stone, beads of glass and angular fragments of bricks or tiles for some special purpose; and it can hardly be doubted that this is to aid their gizzards in crushing and grinding the earth, which they so largely consume.That such hard objects are not necessary for crushing leaves, may be inferred from the fact that certain species, which live in mud or water and feed on dead or living vegetable matter, but which do not swallow earth, are not provided with gizzards, and therefore cannot have the power of utilising stones.
During the grinding process, the particles of earth must be rubbed against one another, and between the stones and the tough lining membrane of the gizzard.The softer particles will thus suffer some attrition, and will perhaps even be crushed.This conclusion is supported by the appearance of freshly ejected castings, for these often reminded me of the appearance of paint which has just been ground by a workman between two flat stones.Morren remarks that the intestinal canal is "impleta tenuissima terra, veluti in pulverem redacta." Perrier also speaks of "l'etat de pate excessivement fine a laquelle est reduite la terre qu'ils rejettent," &c.
As the amount of trituration which the particles of earth undergo in the gizzards of worms possesses some interest (as we shall hereafter see), I endeavoured to obtain evidence on this head by carefully examining many of the fragments which had passed through their alimentary canals.With worms living in a state of nature, it is of course impossible to know how much the fragments may have been worn before they were swallowed.It is, however, clear that worms do not habitually select already rounded particles, for sharply angular bits of flint and of other hard rocks were often found in their gizzards or intestines.On three occasions sharp spines from the stems of rose-bushes were thus found.Worms kept in confinement repeatedly swallowed angular fragments of hard tile, coal, cinders, and even the sharpest fragments of glass.Gallinaceous andstruthious birds retain the same stones in their gizzards for a long time, which thus become well rounded; but this does not appear to be the case with worms, judging from the large number of the fragments of tiles, glass beads, stones, &c., commonly found in their castings and intestines.So that unless the same fragments were to pass repeatedly through their gizzards, visible signs of attrition in the fragments could hardly be expected, except perhaps in the case of very soft stones.