There are certain worms which are free when young, and only become parasites at a later period of their evolution. We will give a few examples.
The Medina, or Guinea worm (Filaria Medinensis, dracunculus) ([Fig. 32]), is the terror of travellers who visit the coast of Guinea; it is common, not only on the western coast of Africa, but also in many other parts of this vast continent, and has been recently found in Turkistan and South Carolina (Mitchell). It was formerly thought that this Filaria could introduce itself directly through the skin as a microscopic embryo; but Mons. Fedschenko, after some observations made on the spot, and corroborated experimentally afterwards by Leuckart, is of opinion that this worm is transmitted by means of the Cyclops, a little
fresh-water crustacean. Thus the parasite is received by means of the water which is drunk; and this remark is the more important since it will henceforth be only necessary to make use of carefully filtered water in order to guard against it. At the end of six weeks, the presence of the animal is revealed by tumours, the true nature of which is not ascertained at first; then some wounds appear, caused not directly by the worm, but indirectly in consequence of the dissemination of its eggs. The Filaria at last is so entirely atrophied that Professor Jacobson, after having seen it alive on one of his patients at Copenhagen, wrote to Blainville: “This Medina worm is not really a worm, it is a sheath full of eggs.” In fact, all the internal organs disappear and nothing exists there except the eggs and their embryos.
Fig. 32.—Young Filaria of Medina; 1, Anterior extremity; c. Mouth; 2, Caudal extremity; d. Anus; 3, Section of the Body.
The Filaria is not allied to the Mermis, as was formerly thought; its organization is different, and its organs become atrophied in a very different manner. The Gordius ornatus, brought from the Philippines by Professor Semper, has given us an opportunity, by different anatomical observations, to correct many errors, especially with respect to the digestive apparatus (Grenacher). The Filaria immitis is a species found by
Mons. Krabbe in a dog which died of a disease to which these animals are subject; it lived in the heart, and twelve individuals, ten females and two males, were found to be lodged there. Mons. Bap. Molin has published a monograph on the Filariæ, giving the characters of 152 species met with in molluscs, fishes, amphibians, reptiles, birds, and mammals: it seems evident that many species have been confounded under the same name.
A small worm, of the size of a slender pin, but much shorter, lives in a manner somewhat analogous to that which we have before described. It is known under the name of Leptodera. In order to find it, we have only to search in the woods for the first snail that we meet with, which is distinguished by its orange or black colour: if we prick with a pin the fleshy foot of the mollusc, we shall see torrents of round worms come out, wriggling like microscopic serpents. These worms also leave their retreat, if we cause the foot to contract by touching it with some acid, or if we place the snail in water. The Leptoderæ are especially remarkable for two fringes which float by the side of their tail, which characteristic suggested the name given to them by Professor Schneider. These fringes so easily fall off, that the greater part of those which have become free have none of these appendages. When placed in fresh or decaying animal matter, in water or in damp earth, these worms, agamous when in the foot of the mollusc, rapidly become sexual and perfect. Thus the snail serves them as a crèche, and the adult worm has no need of external help when it has grown old.
Professor Pagenstecher found at Ostend, on the Nicothoë of the lobster, nematodes which he arranged