The earthworm feeds upon leaves—which are first softened by a fluid discharged over them, and then sucked into pieces small enough to be swallowed, for the animal has no jaws—and upon the half-decayed organic matter which is always present in ordinary soil. The soil itself is swallowed in large quantities; the nutritious portion is extracted, and the undigested matter deposited upon the surface of the ground, near the mouth of the burrow, in the form of castings. As a result of numerous experiments, Darwin estimated the weight of castings thus thrown up by earthworms on an acre of land as 15 tons annually. The following passage is worthy of very careful attention: “When we behold a wide, turf-covered expanse, we should remember that its smoothness, on which so much of its beauty depends, is mainly due to all the inequalities having been slowly levelled by worms. It is a marvellous reflection that the whole of the superficial mould over any such expanse has passed, and will again pass, every few years through the bodies of worms. The plough is one of the most ancient and most valuable of man’s inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earthworms. It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organised creatures.”[39]
The earthworm lays its eggs in a small cocoon formed by the hardening of a viscid material which is discharged by a swollen part of the body called the clitellum, extending from the 32nd to the 37th segments. After the formation of the cocoon the worm moves backwards, and the eggs leave the body by small pores on the ventral surface of segment 14, as this region passes the cocoon. A small amount of food-material is also enclosed in the cocoon, and forms a store of nutriment for the young worms during their early development.
EXERCISES ON CHAPTER XX.
1. Compare the legs of a cockroach with those of a crayfish and a vertebrate. (1900)
2. Describe the respiratory organs of the crayfish. How are they continually supplied with fresh water? (1898)
3. Examine a spider. How many legs has it? Of what divisions does its body consist? Why do you consider that a spider is not an insect?
4. Make observations, and write descriptions, of the habits of spiders, paying special attention to the methods of construction of the webs, the manner of catching prey in different cases, and the care of the young by the parents.
5. Compare a centipede with an insect, pointing out the features of resemblance and difference. (1897)
6. How does a pond-mussel open and close its shell? (1900)
7. Compare an oyster with a fresh-water mussel, and try to find points of resemblance and difference, making careful notes and sketches of these. How many closing muscles has the oyster? Are its gills plate-like? Why do you consider the oyster a mollusc?