Of the other Cestode parasites mention should specially be made of those of Fishes; the vulgar notion that the parasites of these animals are dangerous to man has been shown to be entirely erroneous.
Fig. 9.
Limnæa truncatula.
The Flukes infest animals of all kinds; that which is most dangerous to sheep, and the cause of much pecuniary loss (Distoma hepaticum), is selected here as a type; its structure is shown by a large model, and its life history by a series of diagrams (Figs. 10–13). Here, again, we have a creature which infests two hosts. If the larvæ which escape from the sheep fall on wet ground in or near a pool, they make their way to a small pond-snail (Limnæa truncatula, Fig. 9), into the lung-chamber of which they bore their way. On leaving them the larva may be, and is, too frequently, eaten by a sheep, and makes its way into the liver of that animal, where it causes the disease known as the “liver rot.”
The damage done by the liver-fluke may be imagined from the fact that in the winter of 1879–80 no less than three millions of sheep died of rot in the United Kingdom; this heavy loss is no doubt largely due to the immense number of eggs to which a single fluke may give rise. It has been estimated that every fluke may produce, during its life, several thousands of eggs; and in one case Prof. A. P. Thomas found as many as 7,400,000 eggs in the gall-bladder of a sheep which was suffering from rot, and which, at that time, had in its liver about 200 flukes.
The non-parasitic Flat-worms are shown, magnified, in the upper parts of Cases I. & II. The Turbellaria proper, without any or with a simple or a branched intestine, but without a vent, are represented by Convoluta and Thysanozoon: the general structure is shown by a diagram in Case II., which is here reproduced (Fig. 14). Planaria, Thysanozoon, and Bipalium serve to illustrate the forms of members of this group.
The Nemertine Worms (Nemertinea), with a straight intestine, with a vent, and with a proboscis, may attain to a very considerable length; Carinella and Lineus are represented by large figures, and various species are shown in spirit. These forms, which used to be very unsatisfactory to exhibit, on account of the great difficulty of preserving them complete and uninjured, are now, with improved methods, very satisfactorily shown, as the specimens purchased from the Marine Biological Laboratory at Plymouth prove.
Stages in the life history of the Fluke.
Fig. 10. Egg of Fluke, showing the operculum and the contained yolk-spheres. Magnified 340 diams.
Fig. 11. An embryo forcing its way by its boring-papilla (p) into the wall of the lung of a Snail (e.p.) Magnified about 340 diams.
Fig. 12. A young Rédia (natural size, ½ millimetre or ¹⁄₅₀ inch): pl., pharynx; g, contained germs; p, characteristic posterior processes of the Rédia.
Fig. 13. Free-swimming Cercaria, before the commencement of the formation of the cyst. Magnified 100 diams.
