Should all the conditions be satisfactory, the egg soon begins to show signs of life. It is left to nature to hatch, and the time occupied in the process varies greatly. If an egg-case be opened after some time, a tiny embryo worm, or sometimes a pair, will be found inside, surrounded by a glutinous fluid. The young worm as it grows expands its case, and ultimately emerges—not as a caterpillar or larva, for it goes through no metamorphoses as does the butterfly or frog, the sawfly or even the fish, but as a worm; and now it has only to hasten development and become adult. The adult stage is reached when a girdle has been assumed, just as is sometimes the case among human beings.
It sometimes happens, however, that things do not go well. I have often observed that the eggs of worms are liable to be rendered abortive by the invasion of a smaller worm. It is another illustration of the amusing rhyme about the big fleas and little fleas. Into the life-history of these parasites, however, I must not now enter, as the subject requires a chapter to itself.
It would occupy too much space if I were to detail the wonderful process which goes on within the egg-case. The embryology of the worm has been fully studied, and is replete with marvels. If an egg is examined when the young embryo is almost ready to emerge, it will be possible, through the semitransparent and greatly dilated case, to watch the movements of the worm, trace the current of blood along the elaborate system of vessels, and eventually observe the emergence of the baby Annelid into the world.
For the further study of this intricate subject, I may refer the reader to Mr. Beddard's valuable Monograph, and the works which are there enumerated.
