1. An infusorial embryo; 2. a Trematode embryo having quite recently escaped from the egg; 3. embryo cercaria; 4. fully-formed cercaria, showing alimentary canal and sucker-like head; 5. encysted form of same; 6. Cercaria furcata, with the nervous system and forked tail displayed; 7. in the act of breaking up; 8. tail portion half an hour after division; 9. parasitic worm of another species of Trematoda. (Magnified from 10 to 25 diameters.)
The study of these embryos throws a flood of light upon the obscure history of Cercariæ. After a short period of wandering, their embryos fasten upon the water-snail, and compel it to act as a wet-nurse, and prepare it for a further and higher stage of life. The earliest condition in which I have discovered them concealed about the body of the water-snail is shown at No. 2; in appearance, a simple elongated sac filled with ova or germs, and which in a short time develop into the caudate worms already spoken of; their tails gradually attaining to the length of the mature embryos, Nos. 3 and 4, the latter being a full-grown Cercaria ephemera.
Diesing described no less than twelve species of Cercariæ, some of the most curious of which live on the puddle-snail, in colonies of thousands. All throw off their tails at the moment of changing into a fluke. On placing some Cercaria furcata (Nos. 6 and 7) under the microscope, they were seen to plunge about in frantic attempts to escape from confinement. Suddenly I saw them shed their tails and their bodies divide into two parts, each half swimming about as vigorously as before, quite indifferent as to the severance, and apparently dying from exhaustion. Those represented in Nos. 6 and 7 have a highly-organised nervous system, forming a continuous circuit throughout the body and tail. The mouth is furnished with a sucker and hooklets, which can be projected out some distance, while a digestive apparatus and ventral opening or sucker can be differentiated. The tail is bifurcated and articulated with the body by a sort of ball-and-socket joint, and when broken off, the convexity of one part is seen to accurately fit into the concavity of the other; it lashes about this appendage with considerable dexterity, rarely attaching itself to any of the small aquatic plants.[75]
There is yet another Filarian worm, a pest to the poultry-yard, the Gape-worm, Sclerostoma syngamus. This parasite is widely distributed, and is invested with special interest, since it produces disease, and kills annually thousands of young chickens, pheasants, partridges, and many of the larger kinds of wild birds. The worms find their way into the windpipe or tracheæ, through the drinking water, while in the embryotic or cercarian stage of existence, and their increase is so rapid, the birds quickly die of suffocation. The female gape-worm often attains to a considerable size, and when full grown resembles the well-known mud-worm of the Thames (Gordius aquaticus). She measures full six-eighths of an inch in length, while the male only measures one-eighth. So insignificantly small is he that the female carries him about tucked into a side pocket. The ova sac occupies a considerable portion of the internal body space, and is always found loaded with eggs in all stages of development, numbering some five hundred or a thousand. In shape these are ovoid. On cutting open the windpipe of chicken and partridges, I have found their tracheæ literally swarming with the gape-worm.[76]
A remarkable form of the Trematode worm is Bilharzia hæmatobra of Cobbold, Distomia hæmatobium of other authors ([Plate IV]., No. 102). This genus of fluke, discovered by Dr. Bilharz in the human portal system of blood vessels, gives rise to a very serious state of disease among the Egyptians. So common is the occurrence of this worm, that this physician expressed his belief that half the grown-up population of Egypt suffer from it. Griesinger conjectures that the young of the parasite exist in the waters of the Nile, and in the fish which abound. Dr. Cobbold thinks “it more probable that the larvæ, in the form of cercariæ, rediæ, and sporocysts, will be found in certain gasteropod mollusca proper to the locality.” The anatomy of this fluke is fully described by Küchenmeister in his book on parasites, by Leuckart,[77] and by Cobbold. The eggs and embryos of Bilharzia are peculiar in possessing the power of altering their forms in both stages of life; and it is more than probable that the embryo form has been mistaken for some extraordinary form of ciliated infusorial animal, its movements being quick and lively. We cannot fail to notice the curious form of the male animal, and, unlike the Filarian previously described, it is he who carries the female about and feeds her. The whip-like appendage seen in the figure is a portion of the body of the female. The disease produced by this parasite is said to be more virulent in the summer months, probably owing to the greater abundance of cercarian larvæ at this period of the year.
Fig. 379.—The double parasitic worm (Diplozoum paradoxum).
There are also double parasitic worms, which may be described as a sub-order of Trematoda, differing very much from those previously described. These live on the gills of several species of fresh-water fish, the gudgeon and minnow, for instance. Among them is a most remarkable creature well deserving the name of Diplozoum paradoxum which has been bestowed upon it. It consists of two complete mature similar halves, each possessing every attribute of a perfect animal (a). Each of the pointed front ends has a mouth aperture, and close to it two small sucking discs; while each individual has a separate intestine, consisting of a medium tube and innumerable side-branches. At the hinder end of the body are two suckers sunk in a depression, and protected by four hard buckle-shaped organs. The eggs are elongated, and provided at one end with a fine thread-like appendage (b). In this egg the young (c)—which at the time of hatching is only about one-hundredth of an inch—takes about a fortnight to develop. It is covered with cilia, has two eyes and two suckers; after quitting the egg, the larvæ are very lively and restless in their movements, gliding about and then swimming off with rapidity. If unable to find the fish into which they are destined to live, they grow feeble and perish, but if successful they grow into the Diporpa (d), which is flattened and lancet-shaped, and bears a small sucking disc on the under surface and a conical excrescence on the back. After living in this state for some weeks, and gaining nourishment by sucking the blood from the fish’s gills, the worms begin to join together in pairs, one specimen seizing the conical excrescence of another by its ventrical sucker; then, by a truly acrobatic feat, the second twists itself to the dorsal excrescence of the first, and in this state an inseparable fusion takes place between the suckers and the excrescences involved in the adhesion.[78]
In the group Vermes, the more highly-organised Annelida must be included. These, for the most part, live either in fresh or salt water. The Annelids are various, while the Planaria, a genus of Turbellaria, are very common in pools, and resemble minute leeches; their motion is continuous and gliding, and they are always found crawling over the surfaces of aquatic plants and animals, both in fresh and salt water. The body has the flattened sole-like shape of the Trematode entozoa ([Fig. 378], No. 9), the mouth is surrounded by a circular sucker; this is applied to the surface of the plant from which the animal draws its nourishment; it is also furnished with a rather long proboscis, which is probably employed for a similar purpose.
Planariæ multiply by eggs, and by spontaneous fissuration in a transverse direction, each segment becoming a perfect animal. Professor Agassiz believes that the infusorial animals, Paramæcium and Kolpoda, are simply planarian larvæ.