Leeches are hermaphrodite; and in some genera the acting male inserts spermatophores, or little cases containing spermatozoa, anywhere in the skin of the leech that is being fertilised, and the spermatozoa then make their way through the tissues of the body of the potential female till they arrive at the ovary and there fuse with the ova. In the medicinal leech the mating is said to be encouraged by adding fresh water to the vessels in which the leeches are living.
The eggs are laid in capsules or cocoons attached to some water-plant or buried in the mud, about twenty-four hours after the leeches have mated. The cocoon is formed, as it is in an earthworm, by certain glands in the skin which form a secretion that hardens and takes the form of a broad ring, as it were, round the body of the leech.
Fig. 53.—A Nephelis forming its cocoon and withdrawing from it.
Through this broad ring the body of the leech is withdrawn and the fertilised eggs are deposited in it. The two ends close up, but not entirely, for the young leeches eventually make their way into the outer water through one of the remaining pores. Within the cocoon are six to twenty ova, and these gradually mature and the young hatch out. When they leave the cocoon they are minute, and of the thickness of pack-thread. More than one cocoon is deposited by each leech, but unless the cocoons are anchored to some submerged object they often rise to the surface of the water and float half submerged, and are then apt to be destroyed by water-rats, voles, and other enemies of leeches. At times the leeches themselves destroy their cocoons.
Fig. 54.—Cocoons of Nephelis, showing the growth of the eggs and the issuing larvae, which in the lower figure are leaving the cocoons.
The exact time of the emergence from the cocoon does not seem to be very definitely known, but leeches are long-lived annelids. It is not till their third year that they are of any use for medicinal purposes, and they are said not to pair until they are six or seven years old. They certainly live twelve or fifteen years. But, if we adopt an optimistic view—and in this little book we do—the fact that they grow up so slowly and live so long shows that it will be difficult to replace the shortage of leeches in Great Britain and Ireland during the present war. This could hardly be done by home culture, for even if the war lasts three or four years we have lost the cocoons of the summer of 1914, even if we ever had them.
Leeches have many enemies:— water-rats, voles, the larvae of the Dytiscus beetle, the larvae of Hydrophylus, the Nepa or water-scorpion, the larvae of the dragonfly, and the adult Dytiscus—all feed upon them. Many birds also eat leeches; and it is recorded that at one artificial leech-farm, where there were 20,000 leeches, they were all eaten up in twenty-four hours by an invasion of ducks. Frogs and newts also devour them, and they are not above eating their own brothers. Aulostoma will devour its own species as readily as it will an earthworm.
Those artificially reared, as is usually the case with animals reared in captivity—probably against their will—are peculiarly liable to disease of various sorts. They not only become diseased themselves, but they act as carriers of disease and play the same part to fish which biting insects play to man and other terrestrial animals. They convey to fishes protozoal diseases similar to those that insects convey to man and other warm-blooded vertebrates.